Artem, you always bring a great option.
It's these things that hold Joomla back, because something as basic as logging in with email should be a native option...
1 - I think we should have an exclusive constant, to avoid conflict (which is something very simple and fast and will come in the next update).
2 - Implementing the login with E-mail will take some work because I believe it will be necessary to create a "Joomla Plugin" for this. So it would be good to start a discussion about it, do it calmly...
3 - Note that there are times when we cannot use the "Access plugin".
For example, when I try to access a restricted page, Joomla directs me to the Joomla login page. So if I want to login with email I have to use a Joomla plugin anyway.
Well, the language constant would help in all cases!!