Yeah, maybe it's the OS version. I'm running Catalina and I see the scaling options from that screenshot for my external non-Apple monitor.

Works for me, I'm using two fairly new dell 4K screens, as far as I know it works on any hidpi display

It does for me on both LG and Dell monitors.

You generally have to use the right equipment -- using mini displayport or thunderbolt 3 instead of HDMI, depending on the generation. It's definitely finicky (much like getting guaranteed 4k60 output, especially through a dock)