if it should be standard or not is a judgment call it never worked with any of the laptops
and only a select few iMacs really
Compatible Macs
iMac Model *
Port Type
Compatible Mac Source*
2009 - 2010 27-inch iMac Mini DisplayPort Mac with Mini DisplayPort or Thunderbolt
2011 - 2014 iMac Thunderbolt Mac with Thunderbolt
2014 – 2015 Retina iMacs Thunderbolt No Target Display Mode support
* Mac must be running OS X 10.6.1 or later
https://www.lifewire.com/use-imac-as-monitor-with-target-display-mode-2260906
so the feature you feel should be standard had a 5 year timespan on some select iMac models
seems it was removed when the 4k and 5k iMacs came to be so maybe a technical issue in that department
at any rate
This is a user helping other users with tech questions forum, you are not addressing apple here just fellow users
you can suggest apple bring it back maybe with their new arm based Macs?
using their feedback channel
https://www.apple.com/feedback/imac.html