Unlike the Beta notes imply, the now-missing build # of the client that used to live under the Play button, are not in the Game Settings.
Honestly, this should just be a toggle whether or not to keep, rather than hiding it away in several menus. It’s useful information, to know whether your client has been up to date and also for verifying whether a build has changed or been updated.
Specifically, this is in regards to World of Warcraft.