If you currently use 2-factor authentication via SMS/text msg & “don’t” subscribe to Twitter Blue (fee-based ID validation service), you will lose the ability to use 2-factor authentication via SMS/text msg.
Just grab an authenticator app (Google Microsoft, Authy,…) instead of SMS 2FA and you’re back in business. 
I understand that Elon is trying to recoup costs any way that he can after massively overpaying for Twitter last year, but I don’t think that charging for something that is RAPIDLY becoming the norm in online security is the right direction.
What’s next? Will you have to be a Twitter Blue subscriber to be able to change your login password?