The right clicking on a frame or a selection of more frames and selecting "Remove Frame(s)" has no effect. However the "Remove Frame" button and dropdown on the Animation docker does work.
Confirmed. I have no idea how this happened. It worked last time I touched the docker :/
Whoops, wrong bug, remove frames on multiple frames does work for me on KDE neon.
*** This bug has been marked as a duplicate of bug 392233 ***