Yes! It also exists on iOS, where it’s possible to play both brown/white/pink/fire/seaside noise at the same time as other media

iOS and iPadOS also have that under Accessibility-Audio Visual settings.

I had no idea this existed, thank you!

Is "Dark noise" the equivalent of "Brown noise"?

Yes.

Note: Generally "dark" (as opposed to "bright") means emphasis on the lower frequencies, which is a good description for brown noise, but they aren't formally the same. Formally, brown noise refers to a power distribution that drops ~6 dB per octave, and I have no idea how strictly apple's "dark noise" matches the technical specifications of that.

I can’t quite tell if Apple putting handy utilities (e.g. white noise) and options (e.g. reduce motion) within Accessibility settings, along with controls for braille readers and joystick controls, is part of a universal provision initiative (i.e. reducing stigma in having to use Accessibility settings).

My uneducated guess is that it’s done to avoid clutter for the rest of the settings and because accessibility is a good justification to outweigh this concern.

This is my guess as well.

It's probably easier to get buy-in from stakeholders if what you want to add is hidden behind menu options (reduced risk).

I've been trying to use this since they introduced it, but something on my Mac is constantly turning it off, seemingly at random. Sometimes it'll happen 3-4 times per minute until I give up and turn on a YouTube video.

Only affects the background noise feature, doesn't affect any other sound, music or video. I have no idea what causes it.

Good find! Will check it out.