Can I use Shimano STI brifter with 11 speed mountain derailleur?
No. The cable pull is different starting with 10spd mountain. In Shimano-land you presently do this with a Jtek Shiftmate 8, Wolf Tooth Tanpan, or other actuation ratio converting device. Using a di2 MTB RD with di2 STI also works. In either case there's not a great solution to the left lever being either mismatched or a heavy, expensive shifter sitting there not hooked up to anything. SRAM has three native 1x road groups with non-Doubletap lefts and clutch that get specced on all the bikes that actually come this way.

Edit: Another related setup presently available is the TRP Hylex di2 adaptor kit, which integrates a di2 climbing shifter with the right Hylex lever, which can then control an XT or XTR RD with clutch.