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.