No, more like why you'd use a more expensive filter to hide soft deleted data, instead of just a flag.

Checking whether `deleted_at is null` should be extremely cheap, and it avoids the duplication and desynchronisation of having both “deleted” and “deleted_at”.

Yes, if your database has null. I know this is about postgres, but a lot of stuff is nosql now.

Even in MongoDB, you can can index `null` values, so I don't understand in what database system this would be a problem.