In git the default branch was changed from master to main.
It wasn't. GitHub changed.
In high availability we use primary and secondary, or many other versions of the same idea (main/secondary, etc).
Which is fine because in HA it's about failover, not "who's in control".
There are areas where the master/slave terminology is used because the slave does what the master says. It's "the master has authority" not "if the master dies the slave takes over".