Referring to this dialog:
Though it appears to be working fine with Rhino materials, with non-Rhino materials, I find these behaviors:
Win (6.28.20199.17141, 7/17/2020), Mac (6.28.20199.17132, 2020-07-17): Save as Defaults has no effect; Load from Defaults does not change GUI values in the dialog (on Mac only), and always resets the scene to sphere/checkered/rotate object/simple lighting. Saved defaults are not saved, and sphere/checkered/rotate object/simple lighting are used in the next session.
Windows (7.0.20224.11375, 8/11/2020), Mac (7.0.20224.12006, 2020-08-11): Save as Defaults works; Load From Defaults works, but does not change GUI values in the dialog when you click it, so it appears not to work until you click it, click Apply, and see that your saved defaults are now being used. Saved defaults are used in the next session, as expected.
I think there may be subtly-different behaviors involved with this depending whether or not the 3rd-party renderer is set as current renderer when a material is created, edited, and so forth, but I didn’t infer any obviously consistent behavior so far for such cases.