Target Display Mode was never intended to be a feature, it was an accident and Apple was forced to support it. Macs are computers they were never intended to be displays for other devices. Target Display Mode was unreliable, expensive to setup (relied on expensive cabling) and only worked on a small subset of machines. It was (and still is) smartest to buy an external display and use that for a number or reasons including reliability, ease of setup, low cost of cabling among others. That plus external displays have come way down in price, you can get a decent one for about $100.
Finally Target Display Mode has been dead for 6 years now so I wouldn't count on Apple resurrecting it.