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> So, yes, it's absolutely impossible to get key up events in tty context

There is absolutely no reason that many terminal emulators couldn't translate key up events into an escape sequence and forward them on to the application. So long as they receive them somehow themselves (eg an X terminal emulator will be told of them by X).

It doesn't need to be out-of-band and it doesn't need to involve the tty driver.

The right way to do it is to make them only sent when the application requests them.

For example, this is how focus events work. If the application requests them with SM ?1004, \033[I is sent when the window gains focus and \033[O when the window loses it. There are not a lot of applications that use them, but some can (eg vim).