Start a presentation, switch to Master view via View -> Slide Master, click into a text box, and paste text into it.
=> It's possible to paste text into a master slide.
Normal editing is disallowed, the placeholder entries can only be formatted, thus pasting shouldn't be allowed, either.
Observed using LO 22.214.171.124.alpha0+ (2577d9ecea199ca2c10d852cf279053a1b22faf7), 126.96.36.199 / Ubuntu. In earlier versions Impress crashes for me when I switch to Master view.