It's not clear from these articles how soon damage becomes apparent; I haven't noticed any yet in books which had Post-Its. It may not matter if you don't intend your books to have archival value.
I've tried Book Darts for marking pages, and they can also be used like paper clips to hold notes. I write short notes in pencil, but try to avoid dark leads which make marks hard to erase.
I have at least a few Post-it notes in every book. Some sitting there for thirty years or more. Never experienced any problem. Always used the real Post-it from 3M.
https://siarchives.si.edu/blog/post-it-or-not-post-it
http://libraries.ucsd.edu/preservation/postits.html
It's not clear from these articles how soon damage becomes apparent; I haven't noticed any yet in books which had Post-Its. It may not matter if you don't intend your books to have archival value.
I've tried Book Darts for marking pages, and they can also be used like paper clips to hold notes. I write short notes in pencil, but try to avoid dark leads which make marks hard to erase.