The api.drupal.org website is a good resource and always comes up high in google searches for "drupal 8 [some class]," however, every time I get there, I end up in 8.2.x, whereas I know that I am currently interested in 8.6.x. Soon I'll be interested in 8.7.x.
Some projects handle this nicely by providing a UI for site visitors to select their preferred API version documentation, setting a cookie, and using that stored cookie to always first try to serve the documentation corresponding to that version. I think the mechanics for this are already in place, as there already is the function that when a google search comes in to a 404, api.drupal.org does show a page with all the recent major version numbers as links into "search api.drupal.org for this."
An example out in the world is https://docs.mongodb.com/v2.6/tutorial/getting-started-with-the-mongo-sh...
Closed: outdated
2.0
User interface
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I guess that new features will eventually be implemented in the 2.x branch.
Since Drupal Core adopted semver, and there is more continuity between core releases, always defaulting to the latest documentation is best.