Why can't we make it backward compatible so an interface has each client application say "I support the new interface" if it does? You probably would need to add kernel support to handle the fork/execve case, but I don't think that there is an intrinsic limitation.
fork et al are manage by the shell, not the terminal emulator. And what you're suggesting is exactly what feature flagging in ANSI escape codes achieves. 50 year old teletypes already had way to announce support for new interfaces.
