I don't think this is accurate. Easy Anti-Cheat has supported Linux for a long time, many years before this. I doubt they were years and years out of date, updated, and allowing Linux was turned on by default in the new version. What they likely mean is they ticked the box in this game update to let it run on Linux.

Why would they specifically mention the Steam Deck? EAC doesn't have separate flags for Linux and Steam Deck. It's almost certain they intentionally allowed it to run on Linux. They call out Steam Deck by name because it is a popular handheld that has greatly increased the # of Linux users, not because an EAC update allowed it to work by chance.

And it's still not an active support decision. They allowed it - but it's your business. Whenever people gets problems or successes running through Proton, this is never a publisher's issue.