OldUnreal (Engine)

What's the skinny then? 

FAQ points to this board, but it's seemingly a ghost town here for content about compiling with forward compatibility to UT469.

I see there's a sample package which is getting frequent updates on GitHub, so I'm guessing I just need to "watch this space"?  8-)

Indeed, watch this space. I currently compile ACE with v469 and it is backward compatible with v436-451. We went out of our way to keep all data structures in Core the same (with some minor exceptions). Important data structures in Engine also remain the same (e.g. the base AActor class and anything related to networking). We also kept nearly all of the old APIs, but obviously added a bunch of new APIs. To make sure your mods are compatible with v436-451, you should link them with the old Core.lib and Engine.lib.