El Capitan and newer is still supported officially by Apple. I've been receiving security updates on El Cap. Anything older would not be wise as you state. One thing though. If you are running an older MacPro 1,1/ 2,1 with Pike's bootloader, you are now unable to update El Capitan. The latest cumulative OS update (2018 Spectre/Meltdown patch) rendered it unbootable. So I had to reinstall a pikified El Capitan OS and shut off updates to continue using it. These were kinda strange machines though in that they had 32 bit boot efi. Now some claim this machine wasn't affected but I still am not liking it. I have hidden the update that bricked everyone's pikified MacPro, but it will be risky to try any other updates as well for fear it also makes the system unbootable. Sort of the end of the line for the MacPro 1,1/2,1. It might actually be a candidate for Ubuntu now I'm kinda sad to say. But for a machine from 2006/7... pretty impressive run. Honestly, I think the MacPro 1,1/2,1 is a machine Apple tried to kill off but were not successful. It was just too powerful and upgradeable to fall by the wayside. Mine have dual quad core processors (8 cores) with 32G of DDR2 ECC and GTX 750tis. Hardly obsolete and it does run Windows 10 like a dream... but without a boot screen. But I run everything without a boot screen because I don't want to be stuck with using only supported gpus which are far too outdated. One thing I dislike about Apple is how they limit which max OS X a machine can have. There are ways around it though in some cases. But Apple will never let you know. They want you to buy a new supported device.