[thaumasiotes]

It's called protecting your account. A protected account can only be viewed by followers, and followers have to be approved by the account.

https://help.x.com/en/safety-and-security/public-and-protect...

https://help.x.com/en/safety-and-security/how-to-make-x-priv...

(Note that, because it's always possible to create new Twitter accounts, a whitelist is the only way to prevent someone from seeing your posts.)