They say they exist since before the Federation in the first DS9 appearance, so it's not a retcon to have them show up in ENT and Disco and STID.
Well... yes... poor choice of wording on my part. Not sure what words to use in this case, TBH.
What I was referring to was the fact that we first saw S-31 in DS9, never TNG and certainly not TOS (or TAS), shows that came chronologically before, both in-universe and IRL. It was believable that the DS9 crew had never heard of them because
nobody had. They were invented for that show (despite their long pre-Fed history). Then (skipping VOY), along comes Enterprise, and they're used to recruit Reed (and still, nobody's heard of them). And then they're used in Disco - by this time, they're flashing the black badges around and yadda, yadda, see my last on that - I'm not really all that happy with how they've been used in Disco, if I'm honest. Did not like that
Control business at all, but I digress...
And now, just last night, Ensign Boimler of
Lower Decks was commenting how S-31 uses fast walking in their exercise routine!

It has become, retroactively (hence my semi-incorrect use of the word "retcon" in this context), more of whispered
worst-kept-secret in Starfleet that everyone knows about. Very much the antithesis of what it was intended to be during it's original DS9 inception as an ultra-secret shadow organization that literally
nobody knew about. Its intrigue and mystery have been severely curtailed - much like the Borg since their initial introduction as "a force of nature" in "Q Who?" and subsequent overuse through the decades.
I guess, in the end, that's how it always goes - the writers make a new adversary way too powerful to build artificial drama and jeopardy and eventually need to be throttled back slowly over the course of the arc, in order for the heroes to legitimately overcome them. Same thing happened with the Jem'Hadar, too, come to think of it. One of their little bug ships did a suicide run on the Odyssey in their first encounter and cut it in half, just to show how bad-ass they were. After that, they basically went pretty conventional. IIRC, Stargate did it too. I guess it's a pretty common trope.
So, that's a really super long-winded reason why I used the word "retcon". It may not quite be retroactive continuity, but it's definitely a retroactive
something.
DIScontinuity, maybe?
