I always understood a VCS “master” branch to be in reference to “master” records / the “mastering” process of audio mixing, which in turn is a use of a sense of the verb “to master” akin to “to master one’s courage”—not conveying dominance, but rather the reaching of an apex in skill.

If you’re in the market for a replacement that keeps the metaphorical power, though, I might suggest having a “mother” branch. “Mother” functions similarly to “master” in language, forming compounds like “mother ship”, or “mother sauce”, with very similar connotations: being something from which other elements in the set descend/derive; and to which other elements in the set return for some kind of maintenance or synchronization.