I think that github decided to do that in 2020. And the main reason to do this is that 'master' is agreed to be politically offensive terminology:
Rename offensive terminology (master) - Simon Pieters (kernel.org) <https://lore.kernel.org/git/CAOAHyQwyXC1Z3v7BZAC+Bq6JBaM7FvBenA-1fcqeDV==apd...@mail.gmail.com/#r> -- You received this message because you are subscribed to the Google Groups "sympy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sympy+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sympy/8037a96f-4d4c-48ca-9139-56bec7f5217en%40googlegroups.com.
