Not a solution, a workaround.
Lots of us have come up with workarounds like this, it doesn’t actually give us real support for alternate cuts of movies. You’re specific solution doesn’t work for say Blade Runner which has more than one Extended Cut. You’d then need a sixth movie library. Then there’s the issue with library separation like you have with movies (I have multiple TV libraries and this applies to that as well) causing other issues. For example cast bubbles still don’t cross between libraries.
As others have pointed out there are pretty glaring problems with the way the Split function currently works. In the example you posted with Deadpool 2 and Deadpool 2 Super Duper Cut. If you were to watch one of those versions, they both get marked as watched. If you watch one of those movies and pause it, both versions show up in the Continue Watching area. There are other issues, this thread has like 400 posts about all of them.
I just want to clarify that the request for this feature is not because users have no way to watch the alternate cut. There’s 3-4 commonly used workarounds that still allow you to actually watch the video file within a Plex app. We know about all of them. The issue is that there are pretty glaring issues with all of them. We’d just like Plex to address a pretty common use-case for movie fans in a way that makes sense. While sorting out some of these things might be complex, alt cuts have been a mainstream thing since the late 90s and this feature suggestion has existed for 8 years… it’s time Plex tackled it.
Though it won’t satisfy everyone as others have pointed out, I still believe some combination of exposing the file name / flag in Select Version, Fixing the Watched Status Issue for Splits, and dealing with the Collection conflict issue here (Collection Display Issue - #6 by jeffp1717) would allow a lot of Plex admins to do wonders for making their libraries cleaner. If Plex doesn’t want to tackle a full-featured solution, addressing those three tent poles would at least move the needle for many of us.