The RD-6870 has arrived in stock.
With mechanical drivetrains, the shifter determines the number of speeds,
but with Shimano's electronic drivetrains, it's the rear derailleur that determines it.
I connected just the RD-6870 rear derailleur to a bike that came equipped with a complete 6770 groupset,
and since it "goes forward 10 times and returns 10 times," it's functioning as an 11-speed.
Before this, I've seen several cases of 6770 levers paired with 9070 rear derailleurs, so regarding compatibility... (I'll spare you my thoughts)
With mechanical drivetrains, the shifter determines the number of speeds,
but with Shimano's electronic drivetrains, it's the rear derailleur that determines it.
I connected just the RD-6870 rear derailleur to a bike that came equipped with a complete 6770 groupset,
and since it "goes forward 10 times and returns 10 times," it's functioning as an 11-speed.
Before this, I've seen several cases of 6770 levers paired with 9070 rear derailleurs, so regarding compatibility... (I'll spare you my thoughts)