That is debatable!
You can get throw away numbers as easily as you can emails.
And theres some push back on phone verification due to regional issues or other matters. So it might not be as suitable as you think.
I don’t disagree but, worth noting that the comment that @jbulava made on the 2fa post is relevant about “data/privacy security” on if a 2FA flag coule be added. The same applies to a phone number (even if that phone number itself is not visible.
Some accounts might not have an email, or might not have a phone.
Since you can create an account with one or the other (or both)
And choose not to go to channels that require verification.
Which sure you would deny access to your app from if that option wasn’t on their Twitch account.
TLDR: I don’t expect Twitch to add “is this user phone verified” to the API any time soon. (check the age on the 2fa UV for example)