I'm seeing conspicuous parallels with YikYak. YikYak was facing an abusive userbase that proved difficult to moderate or restrain. Then they introduced the phone number requirement which did seem to reduce abuse. But of course YikYak died not so long after.

Granted, Twitter is in a very different position than YikYak. Curbing user growth in order to afford a better experience for the existing users is likely a good tradeoff.