I have never heard "slave" in discussions about source control ever; especially git. At least not until we decided we needed to change it.
I always thought of master as "law and gospel" in terms of this is the source of truth for the repository. Where the word originates or how it was used in what git was "built from" is not relevant, language changes over time.
The problem with lots of the suggestions, "unstable", "production", etc. They are all specific use cases for specific workflows. Master is not specific to anything but clearly states what the branch is for. Branching strategies in source control is an entire religion by itself.
I do think that "principle" as suggested in the OP is better... or "main", "root", "default"... but I still feel it is unnecessary to move away from a master branch. I won't enumerate all of the practical problems with moving away from master as many others already did a great job at that.
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