Not enforced and hasn't been for over a decade.

And I guess the language is flexible.

It's OK to run a discount for Steam Keys on different stores at different times as long as you plan to give a comparable offer to Steam customers within a reasonable amount of time.

https://partner.steamgames.com/doc/features/keys

Comparable being used when it could have used same. Couldn't find language being explicit on lower prices not being allowed.

Only part I've really seen Steam enforce is that if steam keys are given away then game should have a giveaway on Steam too. So like in the past games would make you make an account on a site then give you a key for free, but now even if they do that, few days later you can redeem the game for free directly from steam without signing up for anything.

Steam Keys shouldn't be given away for free if you aren't also offering the same deal (i.e., give the game away for free) to Steam customers. This includes giveaways for promotional purposes, unless that giveaway is very small (under 100 Steam Keys).