De-Register for 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.