Using 10 speed rear derailleur with 11 speed shifter and cassette 

Facts and figures

Component 10 speed11 speed
Cog spacing3.953.9
Shifter cable pull3.43.6
Pull ratio1.21.1

Calculations 

Each click of a 10 speed shifter moves the 10 speed rear derailleur 3.95mm using 3.4mm of cable pull. An 11 speed shifter pulls 3.6mm per click and to use a 10 speed rear derailleur will need a modification to the pull ratio so it moves 3.9mm (the 11 speed cog spacing). So we need to move the cable mounting point 3.6/3.4×3.95/3.9=1.07. In other words, we need to put a spacer under the 10 speed cable mounting point that will make the lever arm 7% longer.  Arm length is about 31mm, so a 7% increasing spacer would be about 2.1mm thick.

Conclusions 

An extra 7% longer is not a huge amount, so at least on paper it would seem possible to adapt a 10 speed derailleur for use with 11 speed shifters.

Related articles:

Using Shimano 11 speed shifters with 10 speed derailleurs and 10 speed cassettes

The theory

Using a clutch rear derailleur with 9 speed (or even 8 speed!)

What is a clutch-type rear derailleur?

Using 10 speed chain on 9 speed drivetrain

Can you do it?

10 and 11 Speed Cassette Comparisons

I really like the gear spacing on the 10 speed 11-32 and 11-34 cassettes.  When used with a front 32t chainring on a 29er a lot of time is spent riding single-track in the middle of the cassette....

9 speed clutch rear derailleur mash up

There are several ways to get a 9 speed clutch derailleur.  The most popular is to use a Shimano 10 speed shadow plus rear derailleur like a Zee, Saint, SLX, ...

Shimano XT Shadow+ rear derailleur with short cage

There are various iterations of the Shimano XT 10 speed rear derailleur .  However, none are available with a short cage like what the Zee rear derailleur has.