Command-A does select all. On Linux if you use defaults, ctrl+A will do that. If you change your gtk keyboard shortcut theme to ‘emacs’ it will do the equivalent of ‘home’ instead (which is what ctrl-A does on a Mac). To delete a message under these conditions, you can usually do e.g. C-A C-K. But that’s not so suitable for eg a high-risk text box on a buggy site where you want to regularly select all then copy in case the site crashes or loses your data.

Doesn't ctrl+/ select all in either GTK key theme?

Let me introduce you to the Home key: https://en.wikipedia.org/wiki/Home_key

I've always pried this key and other useless keys off my keyboards. TIL.