I have raised a
ticket for inclusion of a similar feature in phpBB core, this allows recovery of user name(s) from email address, and the resulting email(s) have a link back to the password recovery page.
I have created a
PR. If this gets merged to core (and for which version) is outside of my control.
I have set up a test board at:
https://xantiav6.co.uk/forgotusername/ You can create an account there and try retrieving your forgotten user name.
That board is configured to allow email address re-use, so you can verify that case also.