PAYDAY 2 came to Linux officially back in 2016 but as of today they've removed support for it, so it will no longer see updates on the Native Linux version.
They're also bringing the game to the Epic Games Store, although not exclusive, as it will remain on Steam and PAYDAY 3 will also be on Steam. As for why they're removing official Linux support they said this:
Note that Linux users will not receive this update or any following updates coming to PC. In addition, Linux players will be unable to matchmake with other PC clients following this update.
We tried to find a solution, but ultimately found it unfeasible due to the Linux version being on an older version of the PAYDAY 2 engine.
It does, however, still work quite well with the Windows version via Steam Play Proton on desktop Linux and is Steam Deck Verified against the Windows version in Proton. So while it's a loss of support for their Native build, it's still fully playable on Linux.
We've seen this a few times over the years, due to a mixture of reasons but the ultimate reason is pretty much the same as always — Linux and Steam Deck together hold a less than 2% user share on Steam. For many developers, right now, it's not worth the extra overhead to continue Native support until there's more of a market.