Both, kinda?
Like I _could_ probably build a version of the HammUEr specific static library for linux on an ubuntu machine or whatever and create a version of the plugin for linux (altho that'd be More Hoops that I'm not really looking forward to), but the VTFLib dependency is kinda hard Windows right now. I've been thinking about resurrecting my old homebrew VTF reader/writer, but there's some weirdness in UE5 with that right now. Yes, I know there's at least one linux port of VTFLib, but the one I saw was incomplete.
You're also the first person that's ever really asked the _linux_ question?
(The macOS question has been asked I think, but I don't really feel like giving apple a shitton of money for no good reason? so...)
tl;dr: technically feasible, but in reality... probably not gonna happen because I just don't have the time or spoons, sorry