Armello, a grim fairy-tale board game from League of Geeks no longer advertises that it supports Linux and macOS due to their new party system that landed with the cross-play update.
In the news post they mention how there's "No Party Functionality (including Private Games) on MacOSX & native Linux. Due to the severity of this one, we've removed any advertising of support for Linux & Mac from Steam (and shortly Humble Bundle), though folks can still purchase and play those versions.".
Giving a little more detail on their official site it mentions how "The service provider we use for Armello’s multiplayer infrastructure has decided to cease support for their party services on MacOSX and Linux. Unfortunately, this means that Party Functionality, including Private Multiplayer, is no longer available in Armello on MacOSX and native Linux platforms." but they don't go on to name whatever company that is.
For the Native Linux and macOS versions, they're still functional and up to date they just don't have the new party system or private matches. For Linux users though you can use Proton to run the Windows version, which fully works. It also has a Steam Deck Playable rating, which uses Proton now too.
This isn't the only game to go a similar route, with ARK: Survival Evolved recently putting the Linux Native version behind a Beta.