Retail equity ownership, even if it's been higher, was last at current levels in 1999, just before the dot-com crash, according to a new paper by Michael Mauboussin of Columbia and Morgan Stanley. The paper is worth reading in its entirety.

Who Is On the Other Side? A Framework for Understanding Market (In)Efficiency https://www.morganstanley.com/im/publication/insights/articles/article_whoisontheotherside.pdf