Because it is. They are telling paying customers to get stuffed. I was planning on putting Linux on my daughter's PC but with this she will never let me. Only computer left in the house with Windows on it. :(
Just in case there is any misconception: this particular post does not emanate from anyone at Roblox. This is the Vinegar team closing shop and giving context. Vinegar was an open source project which made it easy to run Roblox on Linux.
Roblox Corporation is the one telling a (likely very small) portion of their paying userbase to fuck off.
God discord admins are horrible. Not even allowing discussion about it, what a coward.
Fact is if one of the fears is cheating, the best way to combat it is by releasing an official linux client. Otherwise you're forever leaving an entire base of tinkerers a platform to try and get it to work