Those guidelines refer to what hardware Apple will support, like for spare parts, etc. You might still be able to service the computer from third-party sources. For example, the G4s have been vintage/obsolete per Apple, but you can still find rebuilt Power Supplies on eBay. So, you might still be able to get some hardware upgrades and replacement parts, just not from Apple, but as time goes on this can be more difficult.
Another thing to consider is the support for security updates to Mac OS X/macOS, historically, Apple has generally provided updates to the latest operating system, and the 2 prior releases. So, even if your machine can not run the latest release, you might still be able to get security updates for the prior OS release, but just because they have done this in the past is no guarantee they will follow this in the future.
In Jan 2018 an update was released for High Sierra, Sierra, and El Capitan. Potentially, when the next macOS release comes out, El Capitan support could be dropped. When a known security flaw is discovered, running the older OS with that flaw becomes a risk. Just something to consider.