I believe that has been asked here before... There is a feature request open for that: https://codeberg.org/rimu/pyfedi/issues/949

maybe /domains?

example: https://piefed.social/domains shows a list of domains and banned domains

EDIT: nvm, you meant instances, not domains. oops. it'd would be: /instances like: https://piefed.social/instances

This is now possible to view on the instances page. I redesigned the UI of the filters a bit to try to make it a bit easier to filter by either online state (Online, Dormant, etc.) or by federation status. Let me know if you have any feedback!

The API provides this information: https://freamon.github.io/piefed-api/#/Miscellaneous/get_federated_instances

https://defed.xyz/check?name=piefed.social&software=lemmy