BuddyPress is using the $wpdb->signups table to manage signups since 2.0.0. This table which is only available into the WordPress Multisite DB schema is generated by BuddyPress on regular WordPress installs.
For 10 years we’ve been carrying on creating users to preserve backwards compatibility with 3rd party plugins relying on our “prehistorical” way of dealing with signups on regular WordPress configs, today we are finally deprecating this behavior to improve BuddyPress consistency and fix this 9 years old issue.
If you are still playing with the activation_key user meta and the {$wpdb->users}.user_status (which we used to set to 2 to mark a user as a signup), please update your code so that you now fully enjoy our BP Signups API.
In 14.0.0, we are throwing deprecation notices if you use the BP “prehistorical” way to deal with signups, but please note we will completely remove this code in 15.0.0.
BP Team Updates 