You might as well just write instructions in English in any old format, as long as it's comprehensible. Exactly as you'd do for human readers! Nothing has really changed about what constitutes good documentation. (Edit to add: my parochialism is showing there, it doesn't have to be English)
Is any of this standardization really needed? Who does it benefit, except the people who enjoy writing specs and establishing standards like this? If it really is a productivity win, it ought to be possible to run a comparison study and prove it. Even then, it might not be worthwhile in the longer run.
IMO it's great if a plugin wants to have their own conventions for how to name and where to put these files and their general structure. I get the sense it doesn't matter to agents much (talking mostly claude here) and the way I use it I essentially give its own "skills" based on my own convention. It's very flexible and seems to work. I don't use the slash commands, I just script with prompts into claude CLI mostly, so if that's the only thing I gain from it, meh. I do see other comments speculating these skills work more efficiently but I'm not sure I have seen any evidence for that? Like a sibling comment noted I can just re-feed the skill knowledge back into the prompt.