My Mac mini, that I had to buy to upload iphone apps, was made in 2009 and runs el capitan.
However, I now have to run xCode 9 to upload programs and that needs high sierra, which it won't allow.
Apple have confirmed that
'Because your device is almost a decade old, it is limited in it's options for upgrading the operating system and you have the highest operating system you can install on it'
So it appears it's now scrap and I probably have to buy a new mac every few years to keep uploading their software.
Unless anyone knows a workaround.
Anything to avoid giving Apple more money.