supposedly the programmer layouts have ctrl and alt like old messagease had but i can't find them? all i can find is dessalines saying it should be in the thumbkey developer layouts in here: https://github.com/dessalines/thumb-key/issues/656
I can't seem to find it in any of the layouts, it'd have to be added. But also since thumb-key really doesn't have tap and hold, what are you trying to do?
termux and terminal emulators usually have a bar for these rare press and hold actions.