https://en.m.wikipedia.org/wiki/1%25_rule

Seems like in 2014, a peer-reviewed study confirmed that it's pretty close to accurate:

A 2014 peer-reviewed paper entitled "The 1% Rule in Four Digital Health Social Networks: An Observational Study" empirically examined the 1% rule in health-oriented online forums. The paper concluded that the 1% rule was consistent across the four support groups, with a handful of "Superusers" generating the vast majority of content.[6] A study later that year, from a separate group of researchers, replicated the 2014 van Mierlo study in an online forum for depression.[7] Results indicated that the distribution frequency of the 1% rule fit followed Zipf's Law, which is a specific type of power law.

There's a Wikipedia page about it with all sorts of links to rabbit holes you can go down!