It was deprecated, see https://www.lua.org/manual/5.2/manual.html#8.3
Since it was marked as deprecated in 5.2, it was probably deleted from 5.3.
It might be that the source was updated, but no new LuaRock was published. So you could try to download the source
Then either build that, or take the existing .rockspec file and copy it into the source directory. Then from the source directory do `luarocks make`. Might just work.