SEO content can be any digital material that operates within a content marketing umbrella and serves the scope of SEO. I like to shift the focus from the way they created content in the past, which was focused on one keyword with an optimized anchor text. Today the scope can be served with different content types and not always optimized to have a nice, natural content soup.
Let's take Press Releases (PR) for a moment. The traffic source of PR ended up abused by people looking for fast gains. I remember the numerous gigs and landing pages built around PR that could get you to the #1 spot within two weeks, for example. The catch was the ranking boost, plus the quantity, plus the dream that the PR could be picked up from a big news network. For some people in low competition niches, it worked as a good backlink source for some time and offered some traffic, albeit for a short period. The presence of enough content plus the established PR sites gave a ranking boost in non-saturated niches. Then Google decided to crack down on the PR sites, and most of the hundreds became obsolete. Today if you use PR for SEO purposes instead of business exposure, and without paying much attention to the PR copy and the publishers, you will have negative results.