Recently coming over from a federated 10.9.1 Enterprise deployment and hoping (but not hopeful) to implement organizational-level notifications like Enterprise supported.
Since the June 2022 AGO update, it looks like the typical, \rest & \admin URLs are no longer available save the server specific endpoints. Then again, i'm not really what was available via REST, but I do find it curious that the REST API docs list AGO has having a viable access to the /portals directory (which is where you'd get to your webhook configuration). e.g. https://org.arcgis.com/sharing/rest/portals
I'm accessing via an admin account.
So, does AGO support organizational level webhooks or only service level webhooks? Strange that AGO supports the latter, while Enterprise supported the former and only until recently the latter at a beta level.
Today, ArcGIS Online does not support Organization webhooks (as found in Enterprise). Online supports webhooks on Feature Services and Survey123.
I know they have this request officially logged (I cannot find the tracking reference right now). As far as I know, there is no planned date date to implement Organization webhooks in Online; it's still in the research phase.