No, taking the preparatory course at IIT for PwD candidates is not compulsory. It's an optional program designed to bridge the gap between a candidate's current academic preparation and the demands of the engineering program at the IIT.

PwD candidates who qualify for JEE Advanced but secure a score below the cut-off for their category (SC, ST, PwD) in a particular IIT might be eligible for the preparatory course.The vacant seats in the preparatory course for PwD candidates are filled based on a separate, lower cut-off compared to the regular program.

If you take the course and pass (meeting the minimum qualifying marks), you gain direct admission to the same IIT in the following year for the program you were allotted during seat allocation.

You can choose not to take the preparatory course and appear for JEE Advanced again next year. Scoring above the cut-off for your category in the next attempt will secure you a regular seat in the program you desire.

I hope it helps!