For sure. Though Steam will tell you in the case of the display being small, etc., as part of their compatibility check thing. It will often be yellow instead of green, and it will tell you the reasons why.

I'm talking about games that are full on "unsupported." As in Steam shows the grey circle with the line through it. Specifically recently, Dark Souls Prepare to Die Edition is marked as "unplayable". I skipped past it in my library for about a year thinking it wouldn't work, before randomly looking on ProtonDB and seeing people say that it works fine.

And sure enough, works just fine. Even with DSFix. That's just one recent example for me.