Ensure at least one contactable identity is setup during signup

We have many features relying on email or phone to work, for example, forgot password and in the future, security alerts.
If the user only signs up with username, we cannot contact them.We need to support creating multiple login IDs in the signup flow. Ben has purposed this before, like setting up both email and phone in the signup flow. We would like to enforce a constraint that during signup, we at least create one contactable identity.

The constraint should be upheld all the time. So the user cannot remove the last contactable identity.

The issue names no files or tests. Start by tracing the signup flow and the rules for adding and removing login IDs; map how email, phone, and username identities are handled. Done means signup creates at least one contactable identity and users cannot remove their last contactable identity.

Written by the indexing model from the issue text.