Source on that please? Apple never made a promise to support Macs for 5 years after they are discontinued (for the software).
They DID agree to support hardware for 5 years, and to be able to get repairs, but it has nothing to do with software support.
ASSUMING there is a 5 year lifecycle for MacOS Support for the Intel Macs (a big assumption):
Once a system can no longer get new versions of MacOS, Apple usually releases bug fixes for the last two MacOS versions, usually for another 2 years. That'll bring support up to about 8 years.
Web browsers usually support Mac OS six versions back. For Example, Opera RIGHT NOW supports Mac OS 10.13 (released in 2017). Mac OS 10.13 can work on late 2009 machines and later - or about 14 year old machines.
At a 14 year old machine, most people would be using light web browsing. The limiting factor then would be banking. A quick search of Bank of America's website shows that it requires Google Chrome 93, which was released two years ago, and I think requires Mac OS X 10.10 (per
https://en.wikipedia.org/wiki/Template:Google_Chrome_release_compatibility) . Mac OS X 10.10 was released in 2014 - 10 years ago.
I think that system still has some life to it before it becomes unusable.