I wouldn't say it's in the process of deprecating "master". It's just reminding user that they shouldn't plainly assume that the default initial branch is, or will always be, named "master".
Based on this it does appear to be deprecated, the current favourite is 'main'
There are references in code to `PREPARE_FOR_MAIN_BRANCH`.
You can see some commentary on the mailling lists: https://lore.kernel.org/git/CAMP44s3DExJ-F=MKhKyupr5M0RDvr8k...