I've read that STARTTLS command was added and intended to deprecate the SSL ports. But even major mail servers like google still use SSL ports and even don't allow us to use classic ports with STARTTLS. So it seems situation is quite opposite and good old imap and smtp (at least for authentication and sending e-mails purposes) ports are deprecated. Is it so?
STARTTLS was indeed intended to deprecate the "duplicated" ports, one with encryption and one without, but it is STARTTLS that is being effectively deprecated.
RFC 8314 makes it crystal clear that implicit TLS connections are to be preferred over STARTTLS. It provides specific configuration instructions to mail server operators, including a several year transition period where both implicit TLS and STARTTLS should be offered.