Combining Deore XT 9-speed shifters with Deore XT BR-M8000 (I-Spec-II)
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.