While they can revoke purchases, they've only done that for fraud (e.g.: stolen credit card, chargeback, etc.). I have plenty of games from publishers who went off Steam, unsupported titles taken off the store, games no longer sold due to expired music licenses, etc. -- and they're all in my Steam library to this day.

AFAIK most game platforms resort to just delisting and respecting purchases, with the notable exception of P.T. becoming unavailable for redownload even if you already claimed it[1]. (Except it was a free mini-game and not a purchase) The system isn't even meant to remove owned content to the point that P.T. is still in people's redownload lists, but the download itself results in HTTP 404.

And with Steam, there's always the "option" of downloading your entire library and archiving that. IIRC Steam's DRM is notoriously easy to crack (simple DLL replacement) and things like first-time-setup (additional registry keys, redistributables, etc.) are stored per-game in simple JSON-like files. That wouldn't cover game-specific DRM but it at least covers Steam itself.

[1] https://en.wikipedia.org/wiki/P.T._(video_game)#Removal_and_...