TL;DR Valve has a statement on the use of "ai" in games on the store, one that will enable them to release "the vast majority" of games that use it:
My pessimism is basically that their earlier stance of banning games that used synth-generated assets was just a matter of wait-and-see, a stopgap measure to avoid headaches while they either waited for an official clarification of the rights regime or developed a flag-and-tag system, and it seems they've gone for the latter.
Conversely, the positive aspect of this development is that at the very least there is now a formal rule in place: You must indicate if you're using art or other asset synths in the game you're selling. With luck there will be consequences for not flagging that stuff. It's in Valve's interest to crack down hard on people trying to sneak these things in, and make examples of devs that violate the new rules, even if valve is one of the last entities in the game industry you can trust to put effort into enforcement.