In the last year or two the pace of OS releases has increased as new features and security enhancements are incorporated into the system. Older systems are no longer considered eligible for update and users have been left with the option of continuing to use the outdated and unsupported OS or purchase new machines.
Most of these older machines are still very functional and performance can be tweaked with simple, relatively inexpensive modifications such as increased memory or SSD drives. Although some hardware is capable of running the newer OS versions, Apple will not allow the software to be downloaded and/or installed.
I came across a few articles which all pointed to some excellent work done by dosdude with software patchers to allow some of the newer OS versions to be installed on unsupported hardware.The steps were clear and simple to follow.
So far I have tested Mojave (10.14) and Catalina (10.15) on 2011 MacBook Pros and waiting for a 2009 MacBook Pro for some testing. One MacBook Pro is now running Mojave. Currently working on a 2009 27″ iMac with a GPU problem, and will be updating OSX using these patchers when the repair is completed.