Steam literally warns you for every game. It tells you if you need to be online once or online every time. I don't think you can blame them. If you buy games that require an online activation you can't get upset that you can't play offline.
Example games:
- Always online
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Singleplayer gameplay requires an active Internet connection
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- Online activation
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This game's first-time setup requires an active Internet connection
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I do wish that this wasn't hidden inside of the "Steam Deck Compatibility" section. (There is a yellow box about third-party DRM outside, but for the details you need to click the Steam Deck Compatibility box) But that is my only complaint.
Personally I just don't buy these games.