What? Big Sur supports hardware that is at least 7 years old. Catalina goes back to 2012 (and still gets security updates). For an OS vendor that has yearly updates, they do a pretty good job of supporting older hardware.
True. It looks like the last PPC Macs were released in 2005 and were available through 2006 [1]. The last OS that supported them was Leopard, which got Security updates though 2009 [2].
Even better: Based on apples history High Sierra probably EOLs January 2021 and supports hardware all the way back to 2010. So that's a decade of hardware support which is pretty good.
https://eshop.macsales.com/guides/Mac_OS_X_Compatibility