Some old projects have ' in them, does anyone know why? It’s been confusing me for the past few months now.

It represents an apostrophe (') in HTML. Older comments might have worked differently which is why they still have them.

in the past, those character sequence things worked in comments, but then people found out you can avoid filter etc and they just render exactly as they were written now.

old comments might include them because thats how they were escaped or something