Unfortunately, the changes made to Lua 5.2 will simply break a lot of existing code, in particular the stuff described here:
http://www.gammon.com.au/forum/?id=4960
By removing setfenv, anyone who has followed the guidelines for serializing tables will have their code fail. Fixing this is not just a case of updating libraries.
We would have a massive mis-match of plugins, scripts, functions built into triggers, etc. which would fail for many users.
In March 2018 alone there were 1725 downloads of the older version of MUSHclient (4.94). Based on that I would estimate 20000+ downloads a year. Unbelievably high though that may seem, I don't want to be fielding thousands of complaints about how MUSHclient's plugins or tutorials have stopped working when a new release comes out with a new version of Lua.
The current version of Lua works, and there are no known major issues with it. Fiendish uses LuaJIT in the Aardwolf download, which is a drop-in alternative that would be faster and almost 100% compatible, as he notes.
The downsides are just too great. You are one person that wants extra features. I am one developer that doesn't want to field hundreds or thousands of complaints from other users.
Since the source code is publicly available you are certainly welcome to re-jig the client to your satisfaction, and use whatever version of Lua you like.
You might say "well, people don't have to download the improved version", which would be true. But when a bug is found (as you have been finding recently) then there would need to be a way of making the fix for the old and the new version. This would basically be doubling my workload.