De-Register for Push Notifications I granted permission for a site to send me push notifications on my desktop, and now I would like to revoke that permissions.
While I can change the delivery permissions in Safari to decline push notifications from that site, my understanding is that this simply prevents them from being displayed, but does not disassociate my device from the specific server sending those push notifications.
How can I completely de-register for push notifications from a given website or service, after originally agreeing to receive push notifications?
There is no way to force de registration on server side (as of 2021 at least) but you can disable notifications in the client - which is your Safari settings on your device.

https://support.apple.com/guide/mac-help/receive-pause-or-stop-notifications-mh40609/mac

That tells Apple Push Notification Servers immediately not to pass on any more notifications. The next time whichever service has your device / account hash and sends a notification, Apple will bounce that back to the sender and not bother your client device at all. That completes the de registration since that sender either gets blocked by Apple or processes the halt message properly when it gets a bounce.