Can Librem 5 run Flutter linux desktop apps? Yep! I was able to successfully build and run my BodySculpting weightlifting workout app today on Librem 5 itself. Code once, build for Android, iOS, and Libremt5, how cool is that!
I just followed the official flutter linux install docs doing a manually install by cloning flutter directly from github master: https://flutter.dev/docs/get-started/install/linux#install-flutter-manually