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.
No, you're confused. The next major version after 0.21 will be 22, but consensus changes are not activated in major releases (as they contain lots of extra features that might make some users unable to upgrade or force them to downgrade).
Development works like this:
0.19.0 --> 0.20.0 --> 0.21.0 --> 22.0 --> ...
\ \ \
|->0.19.1 |-> 0.20.1 |-> 0.21.1
|->0.19.2 |-> 0.20.2 | ...
|->0.19.3 | ...
So the minor releases are all forked off an earlier copy of the software and have fixes back ported to them.
The post you were seeing was discussing dropping the "0." in the master branch.