Not in any meaningful, general way on Unix systems it doesn't. Nobody uses psh outside Windows
But nobody is using your hypothetical "structured CLI-based --help-equivalent consumable by LLMs and shell completion engines alike" either. In terms of mindshare, you're starting from scratch either way.
I just remembered docopt, which maybe fits the bill in a more Unixy way, but it and its ports are mostly abandoned, for various reasons.
Maybe they should consider using it.