Hello i was about to ask, what "unstable" version was supported actively by security update, but since I'm a good user i have searched on the Microsoft doc first and there is the answer.
So you will see only stable and beta have active "security update" it don't mean other are insecure, it "just" tell they aren't monitored as much (i post it because like for me some prefer security over anything else and don't (or can't) search efficiently in the doc).
Source : Microsoft Edge channel overview | Microsoft Docs
