Software products can have a surprisingly long life. (Which can also mean a surprisingly long time of providing customer support, too.)
Even the works that are still generating revenue after 15 years, surely most of them have already made 99% of the money they are ever going to make. The people buying rebel assault now are just a few nostalgia seekers. It will add up to a tiny tiny fraction of the total copies sold in it's heyday.
Except the ones (for example Nintendo) who do have a lot of cash, influence groups and lawyers to enforce their rights as long as they want.
Edit: Looks like EA owns it: https://en.wikipedia.org/wiki/Deluxe_Paint#Legacy
I looked past all of this considering the age of it, but I won't fault GitHub for yanking it. I'll host it elsewhere if that happens.