Shortcut key for Pen Pressure

I don't think so but what you can do is make tools presets. Something like a brush say 45 pixels with pressure and one without. When using this brush you can nevertheless adjust the size with the bracket keys.
It all depends of course what type of pressure you choose. (size, opacity, jitter etc)

No - I agree it would be so useful to have a keyboard shortcut toggle for this! - I switch it on and off all the time

thank you Ronald and Mark. I'll try the tools presets method.