So what happens if you want to transmit some data in a nested hierarchical format? Just pack it into some (ad-hoc?) higher level protocol inside one of the flat pieces?
Seems like a recipe for inevitable incompatibilities when two languages no longer have codecs for the new underspecified layered format.
Although the article may not have made it clear, OSC is designed primarily to pass around control signals, rather than arbitrary, non-contextualized data. Control signals have inherent meaning, like "fast-forward <n> seconds", "set color to <x>", etc. The important distinction is that "set color to <x>" (a command) is distinct from "{ 'color': 'x'}" (non-contextualized data).
The layout of controls is hierarchical in the protocol; each control has an associated endpoint, which looks very much like a unix file path. For example, to unpause an audio player, you would send the message "/player/is_playing/set t" (or f, to pause). If you wanted to control the bands of a 10-band equalizer, you could implement that as "/player/eq/band/4/amp/set 0.5" (there are certainly other ways to approach it too).
If you want to send control signals to multiple parts of the audio player at the same time, you could use a "bundle" composed of a separate message for each parameter you're altering. All the messages will then be handled simultaneously.
Because of this, data does not need to be hierarchical. If you really want to, you can consider the endpoints to be "keys" and the control data to be "values" in a hierachical key -> value map, just to prove that everything you could do with hierarchical data is still possible using OSC. But I would encourage you to not think of it in that abstraction because the fact that OSC is built to pass messages, and not non-contextualized data is a large part of what differentiates it from other protocols like json streaming.
Seems like a recipe for inevitable incompatibilities when two languages no longer have codecs for the new underspecified layered format.