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First snapshot of lua-5.4 has been made available in https://www.lua.org/work/ and the page states that "We are getting ready to release Lua 5.4.0, the next version of Lua."
Given how long it takes for us to release new major versions, I'd be inclined to try to get lua-5.4 to freeciv-3.0 already. Otherwise we would be releasing brand new major version (3.0) with lua-5.3 years after lua-5.4 has been released.
Any work on this should first be made to master only, though, and ported to S3_0 only when it works.