> It's possible I actually chose that at some point, for some reason decided I was okay with "by phone number", but not "by email". But that doesn't sound like me, I'm wondering if I unchecked the "email address" one at some point when the "phone number" one didn't exist; then they later added the "phone number" one defaulted to on?

I looked at mine, which I'm sure I've never touched before because I never cared about Twitter settings. As with my Facebook account, my Twitter account was mostly just created to get an acceptable name in case someday I actually wanted a serious social media presence.

Both are unchecked. The account was created in early 2008.