Git took the branch name from Bitkeeper which did have "slave" branches and used a "master/slave" analogy. Git didn't also inherit the "slave branch" concept in that same way, but it did have that heritage accidentally imported when git lazily reused that branch name.
> Does it need to be said that if the US had or has a problem, it's they alone who need to deal with it?
Slavery/indentured servitude is a worldwide problem that still exists today in countries that are not the US. Even if you think this is an over-correction in relationship to the historic US Slave Trade specifically, that was a multinational effort involving the British Empire, the Dutch Empire, and many other Former Colonies beyond just the US' involvement. The US took advantage of the trade, but it neither invented the trade nor was the lone slave owning country involved in that trade, nor was it the last country in that trading group to end slavery trading in practice even if it was one of the last ones on paper.