zlacker

That's only part of it, Twitter is also in the ad business, and in the ad industry, phone numbers are used as identifiers to correlate users between datasets.

If it's just about limiting access, Cloudflare imposes a similar limitation of number of accesses you can have to a website via remote attestation. I think once remote attestation becomes more prevalent, it might become useful in the ad business too, e.g. to prevent you from using ad blockers, or similar things.