1. Big potion of the new generation signup to social media using phone number. they don't have an email. So even with social media connect option they can not sign in to your forum. And they will not be bothered to create an email just to signup to your forum.
2. If you are serious about your forum, SMS gateway is no big deal. It will keep 99% of the spammers away and people start to value their account coz they can not create unlimited accounts just using disposable email.
All the big guys fb, insta, twitter, reddit, whatsapp etc.. all has phone as a unique identifier now.
Having only email and even forcing social media connect users to enter a email is so old school.