amok: It does not matter whether it is DRM free or not. Steam do also not remove games from your library if they do no longer sell a game, neither do any other games store out there that I know about.
This is true.
amok: It is all to do with licenses. If Bethesda (for some very weird reason and a wish to do PR suicide) decides that gOg is no longer allowed to let gOg owners of the Fallout license download the game from gOg, gOg have to remove it from the libraries. Even if it is DRM free or not.
On the other hand, I'm not sure about this -for the same reason I said the first part was true. It might be not the case for each country in the world, but I'm fairly sure that what has been bought legally cannot be retroactively removed. Unless the possibility has been
specifically described in the EULA (see for example DRMs that limit the number of installations) doing this would be a violation of the consumer's rights, so regardelss of the store I think nobody should worry much about that -as long as they carefully read the terms of what they are buying.
For the specific case of Fallout 1-2-Tactics: the copies here were sold with Interplay's terms, and it wold be legally impossible for Bethesda to change them.