Beta features are reserved for paying customers (and in this case you also have to apply to get the IMAP functionality). Free accounts don't get it until it becomes stable.
> The best way is to upgrade your account to a paid account and take advantage of the numerous extra features we provide with paid accounts. [0]
> The best way is to upgrade your account to a paid account and take advantage of the numerous extra features we provide with paid accounts. [0]
[0] https://protonmail.com/pricing