a special "taproot ready" flag embedded in their mined blocks
Is this flag a default behavior of 0.21.1 or do they set the flag manually?
They have to set it manually and Bitcoin Core doesn't set any flags of blocks on its own. Miners use the
submitblock RPC call with the flag set in the block they submit.
There probably won't be a 0.21.1. There's a migration[1] scheduled on Github that will transition from 0.21.0 directly to 22.0. It's named that instead of 0.22.0.
[1]:
https://github.com/bitcoin/bitcoin/issues/20851