More than 3 decades. The original world wide web had them. Link "circles"[1] fell out of favor for SEO around 2015 or so, but before that, they had been a primary driver of Page Rank forever.

I worked in SEO from 2008 until 2015, and developed a lot of tools for increasing your PR from backlink indexing, to running a 15k domain blog network designed to build links to links to links to you, and my favorite: Click Faker - if you were ranked on Google already, on Page one or 2, it would search google find your site, click into it, navigate around, sit on some pages for a while before clicking an exit link or closing the browser - it was very powerful, but nearly impossible to scale, since it needed local residential IPs and I'm against botnets.

1. The circles actually couldn't close if you were looking for the ultimate page rank passthrough, they were actually a line, but still called circles.