Don't forget, one of the things you get for the $99/year developer fee is documentation.
The only hit for the word “documentation” on the linked page is this paragraph which explicitly says you don’t need to pay to learn:
> You can learn how to develop apps for Apple platforms for free without enrolling. With just an Apple ID, you can access Xcode, software downloads, documentation, sample code, forums, and Feedback Assistant, as well as test your apps on devices.
If only it was simple. I had a paid account and lots of test devices. I let it lapse.
Now I can’t test on my own device with my normal iCloud account as I have too many registered devices for a free account, and I can’t access the interface for deleting those old devices.
This edge case has existed since ‘free’ accounts came into existence.
I think even for the new APIs added in non-public betas, it's all on the main website now.
Also there's docs that aren't on the dev.apple site for things like Apple Pay, but it's not a perk of paying $99 a year. I suspect the actual hardware docs for the Vision headset are private too (they were for the Apple Silicon DTK).