Thanks for your reply Paul
I inquired… there is no possibility to update my system to Catalina without losing the use of my nvidia card.
And I don't know if my Mac Pro is compatible with Metal with an AMD card…
But Paul, let me insist please, what I still don't understand is that Modo runs correctly under Hight Sierra, OctaneRender Studio 2020 too (so Octane 2020 in general), but the Octane plugin for Modo does not run on this system.
There is no really way to re-build this version of the plug-in to run on Hight Sierra ?
I don't see what technical difficulty there might be.
I'm sure it would allow all "old" mac users to use Octane 2020.
It looks like planned obsolescence but not because of the hardware but because of the software (or rather of the plugin)
Otherwise I have to change my computer for a PC, but in this pandemic period it really is not a priority…