The gist is that I think it might be unnecessary, and I was only
having problems because I upgraded during a rebuild, or something.

I tried to uninstall icu-data-full to test with luatex, but I
couldn't. Including "not removed due to: [...] xetex", which is what
should happen. This is without this change applied.

At worst it's redundant, but I probably made a mistake and jumped to
conclusions, because I fixed the problem by installing icu-data-full
originally.