I was thinking about this idea when the Zoom saga happened, that's an amazingly quick implementation!

My plan for getting a list of subscribed services was to get people to add a forwarder to their email account with specific keywords (i.e "thanks for creating an account" or perhaps just "unsubscribe" would be enough). This would forward to an API which would check for a recognised service and add it to the list to notify. It has some privacy implications but I think you could narrow the scope enough for people to go for it.

The other method I had considered was getting a list of places your SSO is used from Google etc. Not sure if that's possible through their API but I'm sure with enough of the hacker spirit you could work out a way.

If you go for either of those ideas I expect a lifetime VIP account! (Just kidding)