Lua 5.2 also is obsolete: 5.3 was released in January, 2015. The current version is 5.3.4, released Jan. 30, 2017.
A quick scan of the incompatibilities suggests that the differences are not likely to have a significant impact on Lua programs, at least not what I've written. The implementation inside Celestia might require significant changes, though.
Note, however, that sticking with an older version is not necessarily a bad thing. Lua is used in many "embedded" products, so older versions of Lua have to continue to exist or civilization as we know it would collapse.
To put it another way, once a particular computing environment has been designed into a hardware device, it has to continue to be supported for far longer than the software manufacturers would like to admit. For example, the lab where I work has many extremely expensive data acquisition devices which run "no longer supported" operating systems from ROM. Replacing them all is not an option.