In T375763#10180122, @Esanders wrote:

On a technical side note how does expiration work? Is the account actually marked as "expired" and does Echo prevent generating useless notifications attached to an expired user?

This task should probably be done after T375441: Store expiry date and status of temporary account in database.

Acceptance criteria:

• Echo does not send notifications to expired users
• Trying to mention an expired temporary account should trigger a "mention failure" notification saying that the notification wasn't sent because the temporary account had expired
• Purge all notifications for a temporary account (probably also include ones already read) once the temporary account is expired