To answer my question, new XTR is unfortunately not backwards compatible with "old" 11 speed. 
But I still wonder, could it be possible to use only the new set of brake calipers and Levers XTR BR-M9120 + BL-M9120?
On Shimano compatibility chart, only SL-M9100-I shifters are compatible with BL-M9100/9120 levers.
But there is no mentioning of new SL-M9100-R clamp shifter, that is also available.
SL-M9100-R
Does this mean, that old M9000 XTR Clamp shifter would be compatible with new lever?
On this picture then look basically the same.
If I'm seeing right, there is room between new brace and clamp for normal Clamp shifter, new or old?
Thanks for any help
But I still wonder, could it be possible to use only the new set of brake calipers and Levers XTR BR-M9120 + BL-M9120?
On Shimano compatibility chart, only SL-M9100-I shifters are compatible with BL-M9100/9120 levers.

But there is no mentioning of new SL-M9100-R clamp shifter, that is also available.
SL-M9100-R
Does this mean, that old M9000 XTR Clamp shifter would be compatible with new lever?
On this picture then look basically the same.

If I'm seeing right, there is room between new brace and clamp for normal Clamp shifter, new or old?

Thanks for any help