Shimano 11 speed component combinations
I have found that Shimano offers non-series road flatbar shifters such as the 11-spd SL-RS700.

So this one can be used with GRX-derailleurs, as GRX is compatible to all roadbike components Shimano offers.

The original source (in German) is here : https://www.mtb-news.de/forum/t/shimano-grx-schaltwerk-flatbar-schalthebel.908787/