It is more of an attempt to get real world IDs.
Yesterday I was forced to enter phone number for first time for "login verification". I didn't request any 2FA. They also made it clear they would save the number. Alternative was not logging in. Probably a A/B testing attempt.
I think it's more likely that it's to limit people getting unlimited storage with multiple accounts by limiting it to 15gb per phone number
They forced me to enter a phone number many years ago.
It's stupid how phone numbers have become a proxy indicator for "is a distinct and legitimate human". Do they enforce this in all geographies or are there still places in the world where Internet is available but cellphones and phone numbers aren't common?