Tags are meta-information, and the idea that EN shouldn't support notes in multiple notebooks because you can't do that in real life with paper is absurd. Real paper cannot be searched for meta-information either. Were dealing with a tool vastly more capable than a quill and parchment here.
All modern computer file-systems support symbolic links which is exactly what were asking for, and no, a note with a link in it is not the same thing because that's 3 clicks instead of 1. Symbolic links are a perpetual mainstay because they are inherently useful. Just like notes in multiple notebooks would be.