The problem with your request is that 90+% of the time (with the way I develop), the dead ends are on MRs that got closed or code that never got pushed in the first place. So again, comments as to why this approach is used is way better than hiding it in the history because someone coming to "clean up" code sees the thought process instead of having to remember to search for it.
Couldn't you simply review/bisect at the fork/join points? i.e., take the commits at which forks began or ended, ignore any intermediary commits, and run the bisect (or, read diffs) across that subset? That way you're only comparing at the chapter-markers of the story, so to speak, and not getting mired in the gory details.
Even with `bisect --first-parent`, I still want useful commit messages which "fixup" commits, again, are uniquely terrible at being on the whole.
I do software process and other things, so some of my branches tend to be gigantic (e.g., revamping the build system) and can be 200+ commits because one cannot meaningfully land a build system rewrite incrementally. That one in particular was meant to be bisectable because when rebasing on top of new development, I wanted fixes to be in the "port this library over" commit instead of after some random merge commit based on when I decided to sync up that week (it took a year to do it). So once I get it down to a particular MR, being able to inspect that topic is still a useful property.
Note that this only works with a `merge --no-ff` workflow too. The `rebase && merge --ff-only` pattern and `merge --squashed` are both terrible, IME, at making useful history. The force-rebase workflow is just as confounding to me as the no-rebase workflow (the former de-parallelizes your MR merge process and the latter tends to make a terrible commit history).
Note that even for single-developer projects I run, I tend to make PRs even for my own changes (once it's gotten off the ground).
What you probably want in this case is something like "automatically commit on every change (possibly recording every keystroke)" + "automatically tag based on tests/builds passing or failing" + "allow manual comments at any time, whether based on files changing or not". All of that is technically possible with git/hg/fossil/etc, but it's so much work for both the recorder and the viewer that it's infeasible.
This is especially true if your code is clever. I'm much more likely to understand your polished gem if I can see all the things that you bumped into while you were discovering it.