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If you need an account to play it, it's just not yours to enjoy it.
Same with games. One day a purchase vanished off my PlayStation library, and I must have deleted the email cause it was an old purchase. I still have the save files but they won't refund or give me back the game cause it vanished off their records
I'm quite happy that at least steam doesn't wipe (as to my knowledge) a game from their servers, yes they can remove it from the store front, but if you bought it you can still download and play it
While Steam won't remove whole games, games still get patched and lose content, e.g. licensed music gets removed when the license expires.
Doesn't steam allow you to pick a specific update for your game?
Yes, but that feature requires the developer to mark that version for it to show up in the "Betas"-tab. Steam doesn't allow arbitrary downgrades to previous versions, only to those made available by the developer. And I don't think it would work here anyway, as it would mean they'd still be selling the music, which they aren't allowed to. This feature is however frequently used when it comes to things like breaking compatibility with older Windows versions or other breaking changes.
When the content removal happens in the process of a larger upgrade, the developers will sometimes make a completely new game entry, previous buyers get both versions and new buyers only the new one, e.g. this happened for some GFWL games.
As soon as someone can't be bothered patching Bonnie Tyler out of their 2015 Unity game, you can start expecting games to disappear from Steam's servers.