I don't have time to look through the data today but I wrote a script to collect the usages of `.pth` from pypi.  I realized after I ran it that I skipped source distributions with `.zip` extension but otherwise it's pretty complete:

https://github.com/asottile/pth-file-investigation

There are ~132 packages using `.pth` features (not including setuptools namespace packages which I had to exclude since there were so many of them).  I was planning to classify these but didn't have time to do so.

Some "highlights" from scrolling through the list, two of them are mine (future-breakpoint, future-fstrings), at least one is guido's (pyxl3), ruamel's namespace-packaging appears to use .pth (ruamel.* (12 packages))