> The most popular applications on Flathub still suffer from this - Gimp, VSCodium, PyCharm, Octave, Inkscape, Audacity, VLC are still not sandboxed.

All of these require access to the users data to actually function.

GIMP, Inkscape: edit/view photos and save creations VSCodium, PyCharm: edit/create project files Audacity, VLC: play user media

these apps have the same "problem" in snap requiring --classic to allow access to the users files.