This is not a new feature: right now you can enable 2FA and there is an option to use email as a fallback. If you do not set up any other 2FA option, you will have to login by verifying a “temporary password” that is sent to your email.
I suggest you re-introduce this feature as a new two factor authentication method that’s enabled by default for all users. This way, no user will have to download and setup 2FA – they won’t even know that’s enabled by default!
I’m sure you’ve already encountered what I’m talking about on other sites, like DigitalOcean, that send you a temp code to your email in order to login even if you don’t have any 2FA method enabled.
Defender already has this option. You just have to introduce it as a new feature that’s enabled by toggling a button. Just rebrand it and change some of the wording.
It is very easy to implement by you, it drastically improves the security and us -developers- won’t have to inform the clients/users how to setup 2FA with an authenticator app.