Trouble is that because DISCO has serialized seasons they don’t really have as many opportunities to do unique episodes the way episodic storytelling can allow, especially with only 13 episodes. Can’t have the time to see what’s up with all the familiar races from the previous centuries. We only get a few glimpses into what the universal dynamics are. Hell, we haven’t even seen what’s happened with Klingons (though I have always personally suspected that their exclusion is VERY deliberate).
Yeah it's absolutely deliberate, but I think its from a production standpoint, not a story point. I don't think they have any firm plans for the Klingons in the 32nd century besides maybe some notes somewhere.
My thoughts on the exclusion of the Klingons from Discovery are based on the show's complicated production history, which is tied in general to the complicated nature Trek returned after 15 years.
For folks who forget, the original plan for the return of Trek was a Nicholas Meyer anthology show that would jump around multiple periods to tell stories in unexplored parts of the timeline.The first stop was going to be post-Khitomer as a soft-sequel to the Undiscovered Country. Vaguely recall it taking place around 2300 (this was 7 years ago so my details here could be off). Somewhere along the way, that became the very different Star Trek: Discovery show, that Meyer ended up as a writer for for Season 1, but wasn't brought back for Season 2. But a couple elements of the original plan stayed in place: it wasn't post-TNG but TOS oriented, it used the old Enterprise concept from TMP, and a focus on the Klingons.
Bryan Fuller was brought into be show runner of Discovery (and presumably at the time, head of Trek), and my feeling is he was intending to do a full visual reboot of the franchise for the to inaugrate the modern era, in the way TNG did. What's that about TNG? Well consider if nothing that existed of Trek is TOS, TAS and Movies 1-4. That's it. Now watch Encounter at Farpoint. Imagine the culture shock of a Bridge that looks like a hotel, new division colors, super tight uniforms, a whole new crew dynamic with an older captain and first officer "hero character", no Vulcans in sight and a background "Klingon Marine" And then as Season 1 goe son, very little of TOS-legacy stuff shows up and there is a lot of new concepts and new designs. The Ferengi are built up.
Fuller was going for that level of shift in Star Trek's visual (and so storytelling) language. The focus on Michael and Saru not Lorca,, is analogous to Riker and Picard not being one "hero captain" in terms of plot disruption and no TOS-style "trinity". The new design for Discovery, so far removed from anything else in Trek, analogous to seeing the Galaxy class if your only Starfleet ships you've seen are the Oberth, the Constitution, the Reliant and the Excelsior classes (all that existed pre-TNG). Spore Drive is as disruptive as the Holodeck and easier warping in TOS vs TNG (Captain not calling engine room every warp).
The Klingon design - both costume and ships - was part and parcel of this. Was it out of continuity with everything we saw previously? Yes. Intentionally show. Continuity wasn't the point. If you're aping the visual revamp that TNG (and also aspects of the TOS movies were) over TOS, then you don't care. The first Klingon redesign went without explanation for 25 years, so why not another? I think that is was his thought process. He was looking at it purely through a creative lense in his job to refresh the franchise, not throught he lense of someone intent on keeping consistency with 700 hours of Trek.
But Fuller quickly left, followed next season by Berg and Harberts and most of the writers. Even in late Season 1 there was a hard pivot away from Klingon matters to the Mirror Universe story, before being quickly cleaned up in the last two episodes of the Season.
And then the walk back came, with the redesign of the redesign in Season 2, and the nonsense explanation.Which in turn saw a return to many "Trek norms" that made clear how out of place Discovery was in Season 1. And that in turn was followed by Discovery's best decision, which is leaving the 23rd century entirely to untangle the creative mess it had made.
That mess, I think, was unavoidable. There was no continuity-of-production from Berman-era Trek. Like 2 people came back. It's even in a worse state than TNG at the start in some respect because with TNG, there was 9 years of movie production behind it, many of whom moved over to TNG Season 1. Restarting was always going to be hard, and the visual revamp was well intentioned, if I think misplaced. There was simply far more "institutionalized" Trek in 2017 than there was in 1987. The rules of the universe were really written down in the Berman-era due to the consistency of production staff across the who period. THe weren't ever going to make Klingons not look like Klingons in that period, even during Enterprise. Heck, let's not forget Berman himself nuked the Klingon D-4 class concept for Enterprise Season 1 because it had too small windows. This was 2001, over a decade later, and the EP of Trek was vetoing even the smallest details because it violated his rules for the universe (woudl Kurtzman do that? Hell no).
I think this explains where the Klingons are. Because since Discovery Season 2, I think Trek's behind the scenes production staff has come to realize how much those institutionalized rules mattered to making a Trek make any sense. If anything, I think the example has been Star Wars and the Mandalorian. I think they're paying attention across Town and seeing how Dave Filoni took George Lucas's merchandise-centric mess and with Clone Wars, Rebels, and into the Mandalorian/Andor/ BoBF, shaped it into cohesive universe that kept people engaged because of its cohesiveness. A good example is Death Troopers. First showing up in Rogue One as an original creation, they've since showed up in every other production as the body guards of VIPs.
Trek has started to do this a lot more with Discovery Season 3 (but especially 4), Prodigy, and a bit more of Picard Season 2 and an all-in on Season 3. Now it's deep cuts all around, because it works.
So where are the Klingons? They're victims of being first-to-show up in the Streaming era, and I think Discovery is staying away from them because it doesn't want to go back into "the mess". If they show up in the TNG design, it will demand an explanation. If they show up in the Discovery Season 1 / 2 design, it will demand an explanation. There is no win condition for the show there. Chances are, because of how they do the makeup on that show versus Picard, they'd use Season 2 designs still because that's the "show design", but not the franchise design. If they show up in SNW, I bet they use a new look that is based on the TOS design, and don't use either the TNG or the Discovery design.
My bet? For this reason, we won't see Klingons in Season 5 of Discovery, and we'll never see the Discovery Klingon design ever again. It will be written off as a "show design", and we'll all treat it as a one off, like the TMP Klingons (which are different from the STIIII, STVI and TNG Klingons, which are all different from each other too).
As for an in-universe reason why we haven't seen Klingons, in a theory post about the Temporal War I wrote a couple years back, the idea I had was that the Klingons joined the Federation in the 25th century, but left to fight an adversary in the 28th century and basically fought the war to extinction. In the 32nd century, Qo'nos is a tomb, the system filled with ruined cities,dead planets and derelict ships. And there are a handful of Klingons... maybe as few as 50, spread across the entire Galaxy keeping their culture and history alive as mostly solitary nomads (to better protect themselves) warrior-months... mostly pacifistic but capable of great violence. Basically the philosophy of Picard-era Worf.
Remember what Ezri said: "The Klingon Empire is dying. I think that is more or less the right story to follow. Rome didn't fall in a day. It fell over 150 years. I think the Klingons basically never recover from Praxis. That was the beginning of the end. It lead to a 24th century filled with corruption and decay, capped off by the most destructive war in their history (the Dominion War), which they don't recover fully from. Joining the Federation in the 25th century doesn't stop the decay (it ends conquest). A 28th century shrunken "Empire" - most of its former vassals now Federation members in their own right - make one last play for glory in the 28th century, and get themselves killed off in the process.
Thus ends the Klingon Empire. In the end, Duras would have been the perfect Chancellor for it, because he was the most Klingon Klingon of his time.