It must function simmilar to this block below (it does not work):

<key [backspace] pressed?>

I think this works:

<key (join [] [backspace]) pressed?>

I had seen this works with enter key, not sure if this works with backspace. Just give it a try.

Doesn't seem to work with any non-character keys. It works with +, ;, ç, whatever's on your keyboard, and newline (enter) is a character, but delete isn't, just like shift isn't. I don't think there's a workaround.

say (something dumb)
switch costume to [dabbing v]

Sorry, but you can't do it in Scratch. But you can do it on TurboWarp (https://turbowarp.org) enabling the extra keys addon. You also need to enable “Show TurboWarp keys”.

If you make this on TurboWarp, you wouldn't be able to share the project in Scratch, but you can create a new project putting a link to download the project (on Google Drive or something like that), and telling people to open it on TurboWarp enabling that addon.