As several of you already said, there are many old Macs that still chug along because they are relentless workhorses… I am looking specially at you cMP, but also Mac minis.
Many people still use them for home labs, but I have many clients that still rely heavily on them as "NAS boxes" for backups, file sharing or old OS virtualization for maintenance purposes and software support.
With projects like OpenCore Legacy Patcher this machines have a new extended life, but sometimes the need for older versions comes from the software or hardware that the user needs to run. One example are many old Thunderbolt 1 multidisk boxes (Stardom, RAIDON, Atto, etc…) that need a driver to be recognized by the operating system (even if you configure them as JBOD) and those drivers never made the jump to 64bit in Catalina, so having older versions of ZFS for those cases is a godsend (thank you from the bottom of my heart to lundman).
For these small companies, to keep their old machines in working order is not just a matter of money, but also as a production tool. For example, there is this client of this small printing house where they still use old drum and slide scanners that don't run past El Capitan 10.11, so I install OpenZFS on those machines to have a layer of reliability that I could not get for those old machines any other way.
Another reason to keep those cMPs alive and kicking is the fact that if you need to run a solution in your workflow that involves any type of PCIe card, buying a modern Mac Pro (starting at 8000€ here in Spain for the basic model) is nonsense. One example is to have an archiving station to LTO. Instead of buying a new Mac Pro or an external docking box to plug PCIe cards via Thunderbolt (1000€ or more), you can get a used cMP for 100€ and most of the time you only need to throw in a SATA SSD to revitalize it and there you go: you plug your card in, your LTO tape drive to the card, a JBOD box to the card too, and you can have a backup and archiving station with all the reliability of ZFS on a really cheap solution.
So for me, please, keep those old versions of the operating system supported as long as it is feasible… I wish I had an OpenZFS compatible version for Mac OS X Snow Leopard so I could play with old Xserve I have laying around
