https://static.googleusercontent.com/media/guidelines.raterh...
You can read about what the criteria is, but in short, it directly asks raters to boost established sites and downrank things like forums, personal blogs, and niche websites.
I can see how forums would take a hit because users are anonymous, but personal blogs and niche websites should be rated high. Assuming the blog owner has some expertise to back their work, then just listing a relevant Ph.D. or other professional affiliations on the site about page would be more than good enough to fit these guidelines. This would seem to go more so for niche websites as demonstrated expertise should be more apparent.
Note that site reputation was listed as the very least important metric.
I doubt anyone else read all 168 pages, but can you direct me to the page that supports your claim? I didn't find anything that I would qualify as "directly asks raters to boost established sites and downrank things like forums, personal blogs, and niche websites."
That is pretty exclusive, IMO.
Forums, personal blogs and niche websites are often exactly what I want.
Can you point to anything that might convince skeptical non-Googlers that Google does not do this?
If I were to put my cynical tinfoil hat on, I would say Google has optimized a lot of search results to be favored towards sites which heavily advertise/track their users. Google results are no longer "search term" being matched to "content" but there are additional layers which do more magic, like "is this publisher considered favorable or trustworthy to Google" and "will this result generate a positive ROI for Google".