Where can one figure out which macOS versions are supported by Apple, and in which support stage they are?

I ask because I always assumed that both the latest and the one previous version were supported. As of this writing, the latest version is 10.12 - Sierra. I just happened to land in the Wikipedia page for 10.11 - El Capitan and see the below info. Where do Wikipedia authors get this? Is there a corresponding Apple website to check?

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