currently, the data/prboom-plus.wad file in the release tarballs contains two types of "problematic" content:
1) The menu graphics lumps (i.e. GENERAL and SETUP as well as anything below SETUP, and some more) are composed of artwork taken from the DOOM iwad files.
2) The helper dog sprites and sounds have been taken from the Wolfenstein 3-D game data.
I am pretty sure that distribution of these content is at least "problematic".
In Debian, this problem has already been realized . There, a release tarball undergoes a certain procedure before it can be accepted into the package repository. This includes unpacking the tarball, unpacking the data/prboom-plus.wad file, removing the helper dog lumps, generating alternative menu graphics with a system font, repacking the data/prboom-plus.wad file and finally repacking the tarball with a modified version number . This is both cumbersome and looks crappy.
However, some changes have recently (ahem, about one year ago) been introduced into the prboom SVN to help keep the prboom.wad file free. The most important ones are r4153 and r4154 in which the menus are tought to cope with missing lumps. That is, if the graphics lump for a menu entry is missing, prboom will render the whole menu as text with the internal small font. This looks a lot more consistent than the graphics lumps that are currently generated with the system font. The latter commit applies with some fuzz to prboom+, for the former one a little more work is required.
If the "problematic" lumps were consequently removed from the sources, then this measure would solve #1 entirely.
In commit r4151 to the prboom SVN, the helper dog sprites and sounds have been replaced with modified content from the Freedoom project. Specifically, a smaller and more pale version of the worm daemon is used as a dog, now. That looks less distracting as it sounds and would finally enable Debian to compile the game without "--disable-dogs".
This replacement would solve #2.
So, please consider applying these changes to prboom+ as well to make the whole content delivered in the release tarballs finally *free*!