I accept it is still there being used, but it seems like a child of its time, just the same as notation programs that were built around the physical appearance of the music on the page were children of their time.
Trying to locate the OSC protocol somewhere in the OSI 7-layer networking model, it seems to be down at about level 3, not at the application-layer level 7. Apart from node names (which as mducharme says are arbitrary) and time stamps (which are not guaranteed in any way - there are no requirements for different devices to have synchronized clocks for example) there are basically no predefined semantics to the messages.
It’s not a perfect analogy, but saying “I want Dorico to support OSC” is a bit like saying “I want Dorico to support email.” We all know what email is, but the first response to that request would most likely be “OK, what exactly would you want to Dorico to do with email?” And if the answer to that was something like “Well, when I save a project I want to automatically mail a message to a contacts list stored in the project info, telling people there is an updated version available” the discussion would be getting somewhere, because it is then about the semantics of what you want to do, and the fundamental design of Dorico is all about semantics.
If there are some shared semantics between the OSC data used by different hardware and software devices used for live electronic music, then IMO that is what you really want Dorico to support - not just “OSC”.
The basic difference between OSC and say Rewire or MIDI is that Rewire and MIDI messages do have predefined semantics for the data that is being networked. Of course you can subvert those semantics if you want (in principle you could build a driverless car using MIDI messages for all its internal communication, if you felt so inclined) but IMO the point is that Dorico can “support MIDI” or “support Rewire” without knowing or caring about that.
If MIDI CC1 controls the gas pedal of a car and CC10 controls the steering, so far as Dorico is concerned it is still just MIDI controller data. But with the bare OSC standard, a “message” is just a lump of binary data - even the most basic semantic information to say whether represents an integer, a character string, or whatever is optional.