I got the D850, as well as the MB-D18, a long while ago:) But, I am still waiting for RRS to ship out the updated L-bracket, which I ordered over two months ago... However, the L-bracket for the D810 (with MB-D12) works just fine on the D850/MB-D18 combo. It works really well as a matter of fact.
I have used it since i got the D850, and the absolutely only issue I can see is that the bracket does not easily allow for headphone/mic connections. The USB connection is easily accessible, even with the "wrong" L-bracket.
Just for info...