alt Hacker NewsIIRC, Epic had (still has?) the source for UT4 available as they were trying to push community contributions, mostly on the playable content side but I'm fairly sure they had the core game there, based off what is now a very old branch of UE4. The telling thing to me is if there was a huge demand for it there was no significant effort to pick up the torch, or even do yet another 'spiritual successor'.
The source code repo is still on Epic's Github, not sure if everything else is that easy to get currently. The problem about picking it up by a 3rd party is the fun of getting approval on basically anything.