There were such a thing as slave branches, though; not in git itself, but git was modeled after (and inherited the term ‘master’ from) bitkeeper, which had ‘master’ and ‘slave’ repositories.

I’m not sure that’s super relevant or important, these days, but, it feels worth getting the history right. The term ‘master’ as used in git can be traced directly to a master/slave usage, not a ‘master copy’ usage.