Hi Abhijit,
Maintenance schedule information is deleted from database as soon as the schedule is deleted in USM, i don't think it will be shown/excluded in the SLA reports.
However, there is an option to add exclude period in SLM by manually adding the date and time which in my personal experience is not feasible if you need to pull out report for large number of devices. secondly, tracking the maintenance schedule timings is a task in itself and is prone to human errors.
I suggest you to submit an idea for the same.
Regards
Hitesh