Macos has open port 88 by default. Is it possible to check validity of the password via this port from Linux and how? If the vncserver is active, it is possible to know also the username (by connecting to it with vncviewer) what would make brute forcing easier. Does openssh on macos has something built-in to prevent bruteforcing, assuming it is also enabled?
Yes, this is possible.
Kerberos runs on port 88 on macOS. Kerberos by default has no protections against brute forcing. Submitting an enormous volume of password authentication requests to Kerberos is possible, though to be honest such a thing is useless unless your password is incredibly poor. Kerberos does allow authentication against any user account on the machine so you should consider both the security of your account as well as all your users.
Kerberos is enabled on macOS when you enable File Sharing or VNC to facilitate automatic authentication using your Apple ID on other machines you own. It's a pretty nifty feature. That being said, you should not allow connections to Kerberos from outside your network