High Sierra on a Mac Pro 2008, technically the last officially supported version
of macOS for the Early 2008 Mac Pro is El Capitan. However this does appear to
be a software cutoff point rather than an issue with the hardware required to
run it.
Rebuilding my 2008 Mac Pro, it’s almost 10 years old and has languished for the
last year hidden behind boxes, under powered and forgotten, it was time it had
an upgrade and I fixed some of the destructive maintenance that had been done to
it over the years.
