Withholding user security behind a paywall is unethical
Walling off user-centered security measures behind a subscription paywall is unethical, and will ultimately tarnish your brand as your users get their accounts compromised at greater rates.
I remember using Evernote back in the 2000s, and learning that they will use SSL only if you pay; otherwise they sent your notes in plaintext over http. I found their attitude so offensive that I quit their product completely rather than give them my money.
https://web.archive.org/web/20080701051037/http://www.evernote.com/about/premium/
Evernote Premium | Evernote Corporation
Twitter rescinding SMS 2FA behind a subscription (and not replacing it with an equivalent system like e-mail 2FA) is hostile and reflects how much Elon cares about their users’ security, which is not much.
Remember that compromising access to your twitter account not only gives attackers access to your timeline; it lets them access all your past DMs too.