Exit interview - To decline/accept?

I handed in my resignation end of this week for a role that I loved but my manager was impossible to work with - he harassed me on multiple occasions, was disrespectful, didn't notify me of any clear deadlines, tried to throw me under the bus on emails for shit I never did, took credit for all my work, etc..

I had put in the harassment complaint to HR but they came back with the conclusion your boss shouldn't have yelled at you in front of everyone, but roles in finance are stressful and things like this are bound to happen and both of you are new here. Basically it sounded like they were trying to defend my boss. I also brought up how he is trying to get me into shit for things I didn't fuck up on and if you look at original data or files, you can follow the trail on who it was. HR completely ignored this concern. I asked for summary of what was discussed as response to my email. HR refused.

Ever since my boss found out I complained to HR, he just started going after me more and his way of working is one on one onsite meetings, so he could cover his track I imagine.

HR asked my last day be end of next week despite the 4 weeks I gave in as mentioned on my contract and requested an exit interview. I don't really know what to expect out of exit interview and should I be accepting or declining it? Is it mandatory?

Located in Ontario.