On short, there was an option in dialer settings labeled "Network event notification" that showed when you were receiving a diverted call (shown as "INCOMING CALL (FORWARDED)" instead of just "INCOMING CALL") or it showed when you call somebody and that person is already engaged in a call (shown as "DIALING (WAITING...)" instead of just "DIALING")
This information was nicely integrated within the AOSP in-call UI and was shown on that blue bar that is showing up until call is answered.
There are some hypothetical scenarios to use that, one being the case in which you have two phones/numbers and one is "personal/private" and the second is a "business" one. Say after working hours you will divert the "business" number to your "private" one just not to miss an eventually important call from office or a client. If you receive a call but this function is not in, you cannot tell if the call you're receiving is from a person that dialed your personal number or is someone calling on your work one. If we have back this option we can easily see that while phone is ringing and decide if we take the call or not.
Also when we call someone it's useful to see that the other party is engaged in a call already so we wont keep ringing and eventually get frustrated when the one we're calling is not answering (being in a call, most people do not reject an incoming second call, they just let it ring). We could easily see from first seconds that our call is "waiting" and we could hang up and try again later.
So if it is possible I would like to kindly ask for adding back this feature that we miss since KitKat is out. In the early KitKat builds there was even the option present in the dialer settings (ported from the previous Android version) but it never worked. Eventually in some ROMs it was event taken out, probably the devs considered it either useless or they didn't want to bother with it. Currently there is no KitKat ROM having this function working as far as I know (except if it was included in a stock ROM by the device manufacturer)
Thank you, hoping for a positive feedback...