Alright, I’ve pushed a small change that enables Backspace for deletion of animation frames as well, and I did the same for the collision editor, to be included with Tiled 0.12.3:
https://github.com/bjorn/tiled/commit/25bf5d17e13b84ab9e47593c01a2c160637ae394
There’s still some problem preventing the Delete shortcut from working in the main window, but right now I have no idea about what is causing that problem so I’ll leave that for later.