Not that it matters too much, just caught my attention so wanted to write here. The interface (not just in-game interface) they’re using for Classic is the same one as in retail. You can understand it from small details.
For example, in the login screen, you can see the ‘‘Cinematics’’(plural) button, whereas there is only 1 cinematic in vanilla. The macro coding system is the one from retail; it didn’t work this way in vanilla. Keybindings, in-game interface settings etc. those are obvious ones. In vanilla you couldn’t move character portraits, in Classic you can. In vanilla, in order to use the /r command, the person you’re whispering had to whisper you back as well, otherwise using /r to whisper to that person again who hasn’t replied to you yet wouldn’t activate the command; not the case in Classic. In vanilla, in the chat, when you clicked a certain [General] or [Trade] chat text, it wouldn’t activate that chat command for you, you had to / type it yourself. Now this one I still need to verify: In retail when you got out of a teleported instance (for example arenas), your character’s direction would be the opposite as the one you had before entering the instance (like if you were facing +x , after instance you would be facing -x). In Classic I think they put that as well although I can’t remember if this was also in vanilla. Will update if I remember more.
Oh also, please fix the bug when I login to the game in a new session, the macros have the ‘’?’’ icon on them. Thanks.