Still... There are anticheats that allow Linux, like EAC, Hyperion and many others... If they choose one that does not allow Linux, or choose one that allow Linux but block it, it's a dev issue
Virtually no anticheat worked on Linux just a few years ago except maybe Valve and Blizzard in-house solutions. Games that are out and already committed to a specific anticheat can't do much but to wait, so it is not really on them. Changing the anticheat solution mid-way on a released game would piss off so many people you can't imagine. On a brand new game though, I would agree that this should be considered.
Indeed. What sucks is that it is off by default, I figure most small-time devs simply need to be told it exists. I definitely wouldn't excuse the big players though, most AAA game companies can get fucked for all I care.