Licenses in the source code are always marked.

They wanted to release the Unreal Engine 1 source code. This was not done because not everyone working on the code agreed to it. That's the problem.

There is none. This code may or may not be based on the source code of the PC
(ATI-Rage) version that was leaked in 2022. If it were, it would probably
violate copyright law, but it may also fall under fair use

However, neither the Phantom Edition nor XProger's version come with the source code. Understandably so. The legality of re-distributing the leaked source is questionable at best.

So let's just pretend that the leak was intentional, a rewrite of the source falls under fair use and the whole thing is abandonware anyway:

Writing and maintaining a browser engine is fricking hard and everything moves far too quickly for a single developer now. However, JavaScript is what probably killed TenFourFox quickest. For better or for worse, web browsers' primary role is no longer to view documents; it is to view applications that, by sheer coincidence, sometimes resemble documents. You can make workarounds to gracefully degrade where we have missing HTML or DOM features, but JavaScript is pretty much run or don't, and more and more sites just plain collapse if any portion of it doesn't. Nowadays front ends have become impossible to debug by outsiders and the liberties taken by JavaScript minifiers are demonstrably not portable. No one cares because it works okay on the subset of browsers they want to support, but someone bringing up the rear like we are has no chance because you can't look at the source map and no one on the dev side has interest in or time for helping out the little guy. Making test cases from minified JavaScript is an exercise in untangling spaghetti that has welded itself together with superglue all over your chest hair, worsened by the fact that stepping through JavaScript on geriatic hardware with a million event handlers like waiting mousetraps is absolute agony. With that in mind, who's surprised there are fewer and fewer minority browser engines? Are you shocked that attempts like NetSurf, despite its best intentions and my undying affection for it, are really just toys if they lack full script runtimes? Trying and failing to keep up with the scripting treadmill is what makes them infeasible to use. If you're a front-end engineer and you throw in a dependency on Sexy Framework just because you can, don't complain when you only have a minority of browser choices because you're a big part of the problem.