Reliably detect when someone uninstalls my GitHub App

Background:

I am making a GitHub app with its accompanying website. If a user installs my app, that unlocks some things on the website (incl. for other users). If the user uninstalls my app, I need to reliably detect that event, to lock those things back. However, I can’t seem to find any way to do that.

Main point:
When a GitHub app gets uninstalled, only a Webhooks event seems to be generated about it, the delivery of which I might miss. There seems to be no direct API to query past installation changes (I checked that /app/installations?since= only lists installations that were added or changed, not removed.

My only other hope is to try to get a server-to-server token for a given installation id and then detect an auth error, but I can’t exactly poll GitHub against all my installation ids.

An update: perhaps I can’t even reliably detect when someone installs.
The ?since= parameter seems to be about “installations updated created since”.