Before payment is required, it is possible and not difficult to have an anonymous account using VPN and a burner sign-up phone number.
Discussion
That's extremely unlikely to work. The feds still should know who you are easily since you're still using the internet. It's pretty hard to even get access to a phone number to sign up with and not be compromised.
It's also unlikely that X will ever require fiat payment for all users to keep their accounts.
If anyone is capable of using a VPN and a burner to access a website anonymously, they wouldn't be one of the people with any interest in "Twitter Blue" lol
Zero overlap in that venn diagram
Every user from Mainland China, Iran (they have a big presence), and other countries use VPN as their only means of connecting, and they can post. If the ability to post becomes reserved for paid users only, then that avenue is completely shut out. Sure, those users are a small number compared to normies, but they're an important minority.
Regarding your other point about government (or at least their intelligence/police apparatus) able to figure out who you are, that is not the point. They will need a "lawful" way (it isn't even law anymore, more like decree) to apply social credit pressure, leading to very effective self-censorship.