This an interesting question. Skimming the list it seems that - at least - it is not based solely on popularity (like the Alexa 1 million list for example).

I see lots of websites for small German towns, which should only have a couple of visitors per week.

Of the blogs and personal websites I read - some popular, some less so - only paulgraham.com is there.

I suspect the list is biased to official and very old domains.

My hunch was that it originated well before iOS, and could be traced back to forking KHTML, but if that hunch was right I can’t find any way to validate it.

At this moment, Last-Modified HTTP header reads `Wed, 30 Mar 2022 02:27:02 GMT` (5 days back).

The strangest thing about this list for me is that it contains multiple .onion addresses. It also seems to have been at least somewhat filtered for "objectionable" content (porn etc.).

I assume it must have come from user searches at some point?

My guess actually is that iOS/Apple no longer uses this particular list. I found this by intercepting traffic on an older jailbroken iOS 13 device. Given how old some of the domains in the list are, I'd guess that iOS 15 no longer uses it. I also never actually saw my phone fetch this list, I just saw this URL in another configuration response.

tl;dr; I bet it'll never get updated. (but some similar list at a more hidden/protected location likely will be)

p.s. this list of common spelling corrections is interesting too: https://cdn.smoot.apple.com/static/static_corrections_dict/2...