I set up a Facebook Webhook to notify my app of changes to my business page. Then I did the same with the Messenger platform. Then when I started posting as a user of my page, and sending messages, I found out that the user IDs are different across different pages and apps.
So, facebook provides two endpoints: ids_for_apps, and ids_for_pages. There are quite a few "but"s to use these endpoints, but at least in development mode, and for IDs sent by the Messenger webhook, it works. I get the notification and I'm able too look up the IDs and match to an app id (because maybe the user commented on a photo before contacting me via Messenger)
Now if I try to do the same with a sender_id that I got from the Facebook Webhooks, "feed" subscription, I get the "singular links API is deprecated for versions v2.4 and higher"
The url I try to access, even with Graph API Explorer tool is GET /2.9/{sender_id}/ids_for_apps
What's going on here?