DEHSUPP is a lump introduced by ZDoom that is used to add things to the list of actors and weapons that can be modified by DeHackEd. By compiling a DEHSUPP lump using the associated utility and inserting it into a WAD file, DeHackEd patches can reference things from Heretic, Hexen, and even Strife. It should be noted that DEHSUPP does not do anything by itself, and must be used in conjunction with either an internal or external DeHackEd patch that takes advantage of the new references.
DEHSUPP is now considered deprecated amongst the ZDoom community, as DECORATE now covers the creation of new actors and weapons. The lump itself in ZDoom is now in a text format, but only the one in zdoom.pk3 will be loaded, and if a DEHSUPP lump is present in a wad loaded, it entirely deactivates DeHackEd support for the session.
However, DEHSUPP modding has been embraced by the ZDaemon community, since this port is based on a version of ZDoom that predates DECORATE.