As far as I understand, the feature was removed due to MakeMKV's ability to integrate with VLC:
And this is great for watching discs on your computer, but streaming to other devices is neglected in this affair; in order to achieve this, one needs to open the disc in VLC and enable a streaming setup, then open the resultant stream URL in another player (like VLC for iOS).
- MakeMKV v1.14.0 (5.11.2018)
- Obsolete features:
- Removed UPNP streaming (Use VLC or Kodi with libmmbd instead)
- This requires remuxing the stream - you have to know how to configure VLC's sout and make sure to include all the audio & subtitle streams you want.
- To control the stream (pause/seek), you must be able to control the VLC server from your client device.
- VLC does not currently allow you to play exactly one title from a disc.
Also, I'm not sure if this was in the original implementation (1.10.0 can't be downloaded anymore), but the ability to start the streaming server via the command line would be great!