If admin activation is used, after a new user account is activated the user receives an email containing the following text:

Hello test200,

Your account on "Testing Board" has now been activated, you may login using
the username you received in a previous e-mail.

In actuality test200 is the username, as evident here:

https://github.com/phpbb/phpbb3/blob/develop-olympus/phpBB/language/en/email/admin_welcome_activated.txt

The "using the username you received in a previous e-mail" part can probably be simply dropped.