Combining Deore XT 9-speed shifters with Deore XT BR-M8000 (I-Spec-II) I wanted to convert my trekking bike to a mountain bike, and replace the Deore XT V-Brakes with disc brakes. However, the bike still has a perfectly functional Deore XT groupset (9-speed; unsure about the type).

Googling the BR-M8000 set, I noticed it says it is 'I-Spec-II;, which seems to be some shifter and brake lever mount integration. Will I be able to combine these two or am I forced to replace my shifters too (and thus probably keep with the times and pick a 10-speed groupset?)

Or is this just playing into Shimano's planned obsolescence hand?
I-Spec-II is just some marketing hyperbole for shaping the brake levers and shifters so they fit together nicely. It does not mean you have to have matching brake and shift levers.

You probably should be able to  combine M8000 levers with older XT shifters, unless the XT shifters have a shape that does not work with the levers. Comparing pictures of the brake levers with you existing ones should give you a good idea. If Shimano brakes won't work there are lots of non-Shimano options from the likes of Tektro etc.

Beware that upgrading whole groupsets on older bikes can be problematic as frame geometry standards (bottom bracket width, chainline and rear hub spacing) have changed on modern bikes. If you bike does not have proper disc brake attachment points then really the solution is to go for a whole new bike.