I doubt if this is really OK...?
original valve proprietary Half-Life SDK:
https://github.com/alliedmodders/hlsdk/blob/master/engine/eiface.h
https://github.com/alliedmodders/hlsdk/blob/master/engine/edict.h
someone's half-life reimplementation, which is very similar as valve sdk but relicensed the code to GPL (for the edict.h file even comments are the same):
https://gitlab.com/headcrab-junkyard/ogs-sdk/-/blob/master/goldsrc/engine/eiface.h
https://gitlab.com/headcrab-junkyard/ogs-sdk/-/blob/master/goldsrc/engine/edict.h