Speculating, but presumably this table only needed consultation during creation of a new user session. That’s probably a pretty heavyweight operation to begin with, so adding a scan of a few KB (user IDs being modestly sized integers) for every thousand currently paying users is NBD.

Nope; it was used every time we needed to look up whether someone was a subscriber, which was cached for some things but not all of them.

Self-replying to add: This wasn’t a particularly frequent occurrence.

What sort of events would trigger this lookup if it was infrequent?

It's hard to remember after 13 years, but for example if someone made a purchase and redeemed it, or checked the status of their subscription.

Makes sense :)