Since I can turn my cameras ON via Google Assistant I should be able to turn them OFF as well!
There was a thread about this from last October (link below). I believe the inability to power Off via Google Assistant is by design, perhaps security related.
EDIT: I’m glad you posted this Wishlist request. I hadn’t looked at the Oct '19 thread recently. Having just done so, it appears there’s way to do this via IFTTT (ref. end of thread)