Are the xpdfopen utils still needed? pdfopen should be replaced by xdg-open, and pdfclose was never needed on Unix like systems.
If it turns out that they are still needed, utils/xpdfopen/Makefile.am and utils/xpdfopen/xpdfopen-src/Makefile.am both contain this line:
LDADD = $(X_LIBS) -lX11 -lXmu $(X_EXTRA_LIBS)
Looks like libXmu isn't actually needed. There is neither any X11/Xmu header file included in the sources, nor is there any DT_NEEDED tag for libXmu in the resulting binaries. So the Makefile could be patched, removing -lXmu.
In either case, the x11-libs/libXmu dependency could be removed from the ebuild, as these are the only two programs linking against libXmu.
Ping. Same issue still with texlive-core-2021-r1.