They pivoted away from being an SMS system and that became a legacy part of it, that still worked but wasn't essential.

The part where Twitter was SMS only is like a footnote in its history.

Point is that generally speaking databases rarely delete fields once added, especially field as valuable as a phone number; as such, my assumption is Twitter has had a field in their database for a user’s phone number since it was first released.

Twitter’s been collecting phone numbers since at least late 2006:

https://web.archive.org/web/20061103054924/http://twitter.co...

So what you are saying is that when they started they required phone numbers? As in, what the commenter I was replying to wrote was not true? The 'footnote' in history seems pretty important when the whole point is that ignoring it leads you to assume things that are wrong.