`self.class.discard_column` as a boolean `is_discarded` for "if" and use `updated_at` for "when" about discard

[jarednorman]

I'm not sure that was ever seriously considered. Discard was designed to offer a fairly backwards-compatible experience for apps using Paranoia. Discard was originally developed for use in Solidus and maintaining the same schema as Paranoia made that change much smoother.

Using a boolean column has a major downside; it breaks a common use-case involving uniqueness constraints. Let's say you're implementing a blog and you want the slugs for the blog posts to be unique among non-deleted posts. You can do this with a composite contraint that includes the discarded_at timestamp column and the slug column. All discarded posts will (almost certainly) have unique timestamps, so you can have as many records with the same slug if they are soft-deleted, but only have one where that timestamp is null.

It's also useful to know when a post was discarded in systems where the soft-deleted posts can still be interacted with in some ways.