But it's not clear to me that git master has the same sort of meaning. Why couldn't it have a meaning more like 'master copy'?
master copy
an original copy, stencil, tape, etc, from which duplicates are made
Unfortunately, there are enough concepts in our discipline that take this single source of truth concept and use the master/slave terminology. For an example Google "Replication master" and see if there are any articles on the page one results that don't reference "slaves". They all do for me when I do a query.
I see absolutely no harm in moving the git nomenclature of "master" to "trunk" or "base" some other term. Keep it as an alias and flag it as depreciated.