As I like to joke, the current stuff is Prime, it's TOS that's not.
The Prime Timeline is one in which Spock has "secret" siblings and at some point, Spock looked like 58'year old Leonard Nimoy, one in which the "monster maroons" were worn, and one in which STAR TREK V, VI and TNG "Unification" definitely happened in.
TOS, as we experienced it audiovisually for decades, exists in a bubble outside - or perhaps within or underneath - the Prime Timeline. The uniforms, sets, and technical starship measurements that we've long accepted but were never established in on-screen dialogue are true here, but not necessarily "true" in Prime. The same events generally occurred (unless established otherwise in a current Prime Timeline production), but not the specific way we audiovisually perceived those events.
I could get all MAGoldingesque and posit a detailed system of "Parallels" style quantum realities, but I'll simplify my position thusly: For all practical purposes, the so-called Prime Timeline is all STAR TREK series and films (minus the Kelvin-Timeline ones, of course) since 1982's THE WRATH OF KHAN.
TOS, TAS, and arguably, TMP - what I used to refer to as the ur-Timeline, but instead now refer to as the Proto-Timeline - is, as I stated above, a bubble of its own, consisting of Star Trek as we understood it before various retcons (or "historical clarifications," if you will) introduced by subsequent series and films and temporal incursions. In the Proto-Timeline, Zephram Cochrane is from Alpha Centauri, not an Earthman who moved there later, and the Federation is "a thousand planets and growing" and Gamma Hydra IV is along the Romulan Neutral Zone (not Klingon) and it's the Romulans who don't take prisoners, whereas in the Prime Timeline post-1982, these things are not true . . . or true in the same way.