Last time I messed with Lua (Lua JIT stuff) everything exploded. I really do not want to touch the Lua version. So an upgrade to 5.3. is extremely unlikely.
Background info: I would need a BlitzMax module for Lua 5.3.0 which is compatible with the current Lua BlitzMax module CS2D is using (PUB.Lua). As far as I know there is no such thing (BlitzMax is dead and not updated anymore) and I'm not willing to invest time to upgrade it on my own (it's an EXTREMELY annoying task).
Engineer ‘s utf-8 Lua have some big Bug and hook:say dont support chinese,
I think if CS2D support UTF-8 completely，It's will not have this trouble.
Lua 5.2 support utf-8,so I think it's a good way！
Upgrading to Lua 5.3 is useless, it just minor update. And also it will breaks the game's code, like shooting, spectating, EVEN script function wouldn't work because of this. Think twice before deciding something.
I think CS2D staying at 5.1 is the best thing DC ever did because I develop a code injection that replaces CS2D stock Lua 5.1 with more powerful LuaJIT (see signature) and if CS2D changes its Lua version, it means my project no longer works because Lua 5.1 and 5.2 is not compatible between each other.
I wrote Lua code optimized for LuaJIT for living and I can say LuaJIT is all what you need to extend your server capabilities beyond what CS2D offer.