Many masked packages in x11-proto/ (they are consolidated in x11-base/xorg-proto) do not get removed with "emerge --depclean" since the package was not revbumped. Manual rebuild of package is required. Do other versions of the same package require a revbump, too?
I'm not sure why it would require rebuild, since this was only a DEPEND change and not RDEPEND.
emerge -avc x11-proto/videoproto gives me: x11-proto/videoproto-2.3.3-r1 pulled in by: dev-qt/qtmultimedia-5.9.5 requires x11-proto/videoproto