Also I could not find information on why 1.20 was skipped from public release. Where can I find this information? Thanks.
I don't know, but 1.20 and 1.21 were committed within about two minutes of each-other. I think the commits for 1.20 must have been published to GitHub, then the maintainer must have realized an updated 1.21 was desirable to publish instead, with more commits (some CI/test fixes and code fixes).
What are the changes/fixes/updates?
I recommend cloning this repo and looking at the commit history. For example, running the following if you have gitk installed: gitk v1.19.2 v1.20.0 v1.21.1
Here's what the commit history looks like:

I'd note the v1.20.0 git tag was missing two commits from v1.19.2, so that might be another reason why 1.20 wasn't published to the npm package registry.
Next release, we'll have something figured out here.
@olingern 1.22.0 is out and there's still nothing in the changelog referencing that version…
The changelog needs to be updated for yarn version 1.22.0.
Related issue: https://github.com/yarnpkg/yarn/issues/7735
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@olingern
1.22.0is out and there's still nothing in the changelog referencing that version…