SuperTokens is an open source authentication solution offering features like: Different types of login: Email / password, Passwordless (OTP or Magic link based).

SuperTokens.com

Hey, SuperTokens team. I have a lot of subdomains and all of them should look at the same user pool, but If customer signs up from a.mydomain.com it should not have a user at b.mydomain.com. For now I'm overriding the signup/in flow by modifying customer's email to original_email+subdomain@customer.com. Is there any other way to achieve the same functionality? Like changing user identifier to something else than the email.

The way you are doing it now is the best way given the current feature set

We are going to release multi tenancy soon using which you can make each sub domain it’s own tenant. Which is a cleaner way