So I e got some 2014 XT brakes that came on a trek Remedy 9 and I need a new rear shifter.

How do I know if I can use an Ispec shifter? Or do I just get a clamp one?

I’d rather the space be freed up for the reverb.

You should be able to fit Ispec B to those.

I just found a review of the bike saying that they wished they’d specced Ispec to the bike.

Is it these XT brakes you have? – if so then they are ISpec B compatible

You can fit this XT Rear I-Spec B 10speed shifter but if you have 11 speed on your bike then you’ll need to buy this XT M8000 I-Spec B 11 speed shifter

Mine are m785 – b

I assume the b means Ispec B right.

Yep, make sure and get the corresponding ISpec B shifter and you’re good to fettle.