To add to @rjs's answer in which he referred to MA's special Covid rules for telecommuters, at this point those rules apply only to the 2020 tax year. MA has not yet issued any special guidance for the 2021 tax year.
The 2020 rule was that, if a non-resident taxpayer was physically working in MA prior to the pandemic, and then began telecommuting from out-of-state due to the pandemic, their income would continue to be regarded as sourced in MA, and thus taxable by MA.
https://www.mass.gov/directive/directive-21-1-personal-income-tax-guidance-for-employees-who-telecom...
The normal rule is that an MA non-resident would only be taxed by MA on income from work actually (physically) performed inside MA. Thus, under normal circumstances, the income of a NYC resident who worked 100% remotely from a NY location would not be subject to MA taxation. (It would, of course, be fully taxable by NY.)
Presumably the normal rule will hold for tax year 2021 unless new special guidance is issued.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
