In previous releases of the client software, if the server or realm you were on was running an older version of the software, but still in the same major version number, you could still connect. For example, the server or realm was on 1.14.20, but your client was just updated to 1.14.30.

With this latest update, you are unable to connect if the server or realm is not yet updated. This is a huge problem for servers, as the server software is not yet updated.