Since the manual is not updated, I'd like to ask:
How do you use the 8-player joysticks mentioned in the 0.12 change log?
Are there any keys assigned to btn(b,index) with indices higher than 1?
Can we change those keys?
How are we to use this feature?

I believe they're mapped to gamepads right now. I can't test that because I only own one.
Changing those keys is on the roadmap, but I don't think it's possible at the moment.