I believe those would be the 2010 iMacs which as far as I know do not support Metal, only the 2012 iMacs and later support Metal.
The official information from Apple on which Macs support Metal can be found here:
This list can also help match the GPUs to the model year.
I realise that OpenGL isn’t supported anymore, and for good reason.
But I’m having a few users complain that they’re getting the “OpenGL not supported” message even though their machines should support Metal according to the appropriate lists.
I’m not a Mac user but I’m in good contact with one of these users who reports having this problem on “two computers (one is a 21 inch with an i3 and Radeon 5670 the other is 27 inch with i7 and radeon 5750)”. He has also sent me the .spx files for each machine if that’s any use to somebody at Epic.
Thanks 0x414e54, I can go back to the users with this info and see that it tallies up.