Yup, I ran MtG: Arena since public beta on Linux (way before the Steam release) and it was fine. I would have to manually reinstall every update until someone fixed it in the Lutris script, but the installation process was totally fine. I also installed a bunch of games before Steam was even a thing on Linux and they ran just fine in WINE (old PC game Lords of the Realm II, Starcraft & Starcraft 2, and others).
The Deck helped more with popularity & spread of Linux, rather than actual compatibility.
While true, it represents a greater investment by Valve into gaming on Linux. Before the Deck, there was a steady improvement, but great strides were pretty rare. After the Deck, there was a ton of fixes that landed, especially during the 6-12 months leading up to the launch and the year following (i.e. as backorders were being fulfilled). So they absolutely invested more resources when the Deck launched.
And those unsupported titles are almost all competitive multiplayer games. Regular multiplayer titles that are mostly PvE focused work usually fine under Linux.
It’s still hit and miss, but a lot more hit than miss these days. I still run into older games where the Unsupported flag really does apply, but more often than not, Unsupported just means I may need a small tweak to get it to work.