Singer/guitarists having a "cheat sheet" taped to the top of their guitars have been around for decades!
It's an "Aide Memoire", more likely a "song list", than a set of words to everything they are going to sing!
For Gawd's sake, there's only so much room on the top of of a guitar.
I've always been of the opinion that you can't do a song justice if you have to read it as you sing it.
If you can't commit it to memory...it's not the song for you!