Earlier, thought to document how to do this by creating a role, but this is only available to people with lots of permissions, and messy - many one-person Roles - and doesn't work for, e.g., the private profile use case.
In migration of release where private cards stop working, do one or more of these:
Warn in the upgrade notes of the change.
Code that warning to show up when the migration script is run, if there are any cards with permissions restricted to "Me".
Ideally, migrate existing private cards to the new method.
actually, the new permissions system will cover this kind of a thing, but there won't be a separate concept of "private" cards, and we're long past migrating, so I'm going to close this.