[giraffe_lady]

You put the "is not null" on the index itself and then simply don't use it for the much rarer queries that are on deleted accounts. Or just use a view for active accounts.

Overloading timestamps to carry a boolean on null is awesome as long as you decide that's what you're doing and use one of the several standard techniques to dodge the easily avoided potential downside.

This isn't a valid security concern, more than any other incorrect sql query would be anyway. A dev can always write a bad Q, you need another way to address that it's not more likely here because of the null.