Drupal user login options - username and/or email address
There are many and various use cases of how you would allow your users to login into your website. Most will require a username, while some requirements will allow only the user name OR just the email address. With Drupal there are many ways to do any of these thru Drupal modules.
To login using username only
Currently, the default functionality is provided by core Drupal 8 and Drupal 9. There are discussions on Drupal 9 to allow either username or user email address options. See discussions here https://www.drupal.org/project/drupal/issues/111317
To login using email address only
Allow users to register using an email address. Users can then log-in using their email address and password for authentication. There is an active patch that allows username login as well. See discussion and patch provided here: https://www.drupal.org/project/email_registration/issues/657472
Module project page: https://www.drupal.org/project/email_registration
To login using either user name OR email address
This simple Drupal 8 module allows users to log in with either their username OR email address from the same input box on the standard login form (it turns the "name" field into a username OR email address field).
Module project page: https://www.drupal.org/project/login_emailusername