It wasn't an oversight to make the features optional. They're deliberately optional so device manufacturers aren't forced into a ridiculous all or nothing situation.
At a certain point I wonder if you just declare both 2.1a and 2.1b with different required subsets of the "full" 2.1 functionality. Or, like, 2.1 requires only the basics and you simultaneously declare 2.2 which has the extra stuff. Remember 802.11 b/g/n routers, and how they're up to, like, ax now? Slightly different situation, but I'd call it a reasonable demonstration that consumers can handle there being four or five different versions of a single standard, as long as those versions are narrow, clearly defined, and well communicated.
I think they would have been better off forcing manufacturers into that situation, and that the feature list has grown so large that it’s no longer a sensible specification for purchasing decisions, which will erode consumer trust.
It's kind of going in that direction with HDMI and DisplayPort over USB 3. But.. not exactly because they're not actually encoding video data into USB packets. It's too difficult to have abstractions like that when you're dealing with such insanely high data rates.
The parts that are necessary to make a screen run at full capacity very much shouldn't be optional. Like taking input at native resolution, native bit depth, max native framerate, no chroma subsampling.
