if it all depends on whether you took a deduction on your tax return for the net premiums paid before the yhis rebate.
if no deduction it is not taxable
if you took a schedule a tax deduction for the net premiums then it's miscellaneous income reported on schedule 1 line 8z in the year received
couldn't find anything specific on this but to me it seems appropriate if you took a self-employed health insurance deduction this should be treated as a reduction of this expense in the year received
see this link Q15
https://www.irs.gov/newsroom/medical-loss-ratio-mlr-faqs#:~:text=For%20a%20cash%20rebate%20paid,more...
