I have been on LTD since 2016 after being diagnosed with MS. If my employer terminates me for frustration of contract, do my LTD benefits still continue?

At some point, coverage under your policy would be terminated as well; however, under the vast majority of policies, your existing LTD claim would nevertheless continue beyond the end of coverage, as long as you continue to pass the Total Disability test. So yes, your LTD benefits should continue.

That said, if you returned to work (past the recurrent disability period, if any), and then went back off of work again, such that you needed to start a new claim, then you would no longer have coverage for such new claim.