I think GET /api/v3/resolve_object should work:

curl --url-query q=https://feddit.org/post/2401677 \
    https://lemmy.ml/api/v3/resolve_object \
    | jq .

(note that the value of q is url-encoded by --url-query)

You can get 50 items at most with limit=50. Try

https://lemmy.ml/feeds/c/lemmy_support.xml?sort=New&limit=50

Not a direct solution but GET /api/v3/site may help.

Cheese Day

Maybe some rules in nginx.conf has been delegated to nginx-internal.conf.

I suspect your instance was used to backup the original communities.

You shouldn’t post the auth value here - it works like a username and password.