I've looked for an rss feed link for the Ansible Community release (not ansible-core) that I usually install with pip, but could never find.

I should have looked on pypi.org first, from https://pypi.org/project/ansible you can find Release history tab, which gives this link that can be subscribed to:

https://pypi.org/rss/project/ansible/releases.xml

For ansible-core, visit either https://pypi.org/project/ansible-core/#history (the rss link being https://pypi.org/rss/project/ansible-core/releases.xml) or find https://github.com/ansible/ansible/releases.atom from https://github.com/ansible/ansible/releases.