I am not sure where you are going with this or indeed now your concern with my original post.
90% of the stuff we see on
https://osgameclones.com/ (for those playing along at home it is a site gathering all the "legal" takes on commercial games with source code released,
https://en.wikipedia.org/wiki/List_of_commercial_video_games_with_available_source_code is wikipedia I know but a reasonable covering for some of the leaks, accidental disclosures and retracted versions that osgameclones intentionally does not deal with) is usually "game has been out for 5 years, has not become an esports phenomenon/multiplayer game of choice, sales are barely enough to pay the accountant to care about it/have support options, let's release the source code so the kids can port it to everything for us (maybe also fix some bugs) and we maybe get a boost in whatever download options we have available and some good PR for the sequel that just so happens to be coming out, however all the art will still be owned by us so we can funnel people to the download shop and we also dodge bad PR because someone put the art in a porn game and we technically have to allow it". The other 10% being serious reverse engineering efforts, engine ports (unreal engine game can probably run in similar vintage unreal engine game with some work, doom - doom, quake/iD - quakeID...) and those vanishingly small number of things that release code and assets as a permissive effort.
As a shorthand I then noted assets as being straight up copyrighted as a shorthand for no rights over baseline fair use (
https://fairuse.stanford.edu/overview/fair-use/four-factors/ ) or whatever is available under machinima/let's play/content usage/footage usage agreements (most of which say no hacked/modified versions, though that tends to draw more from trademark aspects for the things I saw). While creative commons still utilises copyright (save for CC0 which is effectively making it public domain) is it never the less under the banner of copyleft or permissive licensing, something most tend to meet in open source code (
https://itsfoss.com/open-source-licenses-explained/ ) but as things intended for code don't work for artwork most instead look to creative commons or its competitors (surprisingly few in number and notability compared to source code stuff).
As a technical feat I could happily make a game that even mugen types (again for others playing along at home it is a fighting game engine notorious for having any number of characters ripped and remade from other games -- if it is big enough to have ever had fan art it is probably there, multiple times, with some original stuff on top of that. Saltybet is probably the most notable take on this but that is getting off topic) would look on for and nod head in appreciation at the scope achieved. I would however expect to have copyright and possibly trademark concerns to appear in fairly short order. It would also have a knock on effect, usually a very chilling one, for getting people in on a team to help make it, distribution of the eventual game/mod and more besides which this should serve to dodge (give or take those that want payment**/profit sharing as commercial uses are not allowed by the looks of this one).
**I suppose technically I could pay for services rendered out of my own pocket, advertising on the website and ebegging platforms that just so happen to support a group (the game being free regardless) are potentially also on the cards.