It's my understanding that the "doc" USE flag is "usually" not for man pages but for other docs (otherwise, it's a default flag like for nftables since users expect man page to be present). mpv-0.29 used to have dev-python/docutils (for rst2man.py) in DEPEND and enable man-page build by default. Right now we have: $(use_enable doc html-build) $(use_enable doc pdf-build) $(use_enable doc manpage-build) I also suggest removing the rst2pdf dependency and disabling pdf-build (it's the reason I did not want to specify "doc" USE), mpv is the only package in all of gentoo main repos that has a dependency on this old python2-only package (with many dependencies of its own), and I'm not sure who wants a pdf of mpv docs. By removing rst2pdf dependency, making "doc" a default flag would probably be fine but people who just want the man page may not want the html docs (can always just mask those though). I personally think reverting to 0.29 behavior would be best unless really want to make docutils optional.
This -dev ML thread may be relevant? https://archives.gentoo.org/gentoo-dev/message/a6a8e840773da77ee6008ac23b4f89e1
Created attachment 602598 [details, diff] Install manpages unconditonally like it once was.