ZDoom

Like Graf said, redefine STARTUP1 through STARTUP5 in the LANGUAGE lump, and they'll be dumped to the console near the end of the loading sequence.

This already exists. Boom added 5 empty strings for outputting custom text at startup nearly 20 years ago.

I suggest build in opportunity of custom message that appears at loading/compile, how correct name it?, time.
That can be used as for silly messages like "keyboard not found... press any key", and for more serious thing like "addon will not work without %mod_name%/this mod will work only in gzdoom version 2.435.435.random_number/this mod will not load with brutal doom/etc.". All that messages will write mor creator of course.
Yes, I know for this exist readme, but this as EULA. How many of you read EULA when install photoshop/3dmax/winamp/other program?