State tax filing

I'm not an attorney so I can't comment on the legal issues, but rather how New York has applied the doctrine.  That one case you cite is well known and is the "outlier" when it comes to New York's telecommuter policy.  Since in that case there was no real office in New York, and since the taxpayer did no work in New York, New York couldn't tax him even with their "convenience of the employer" doctrine.

When a New York company has a New York nonresident employee doing work for them, often times New York requires that the company include all of the income on Form W2, and then it is up to the taxpayer to allocate which of that income is earned "without of New York" by using Form IT-203-B:2019:Nonresident and Part-Year ... - Tax.ny.gov (PDF attachment you can open).  On that form, pay particular attention to the line 1j.  You will notice that the preceeding lines ask how many days (while working on behalf of a New York company) were worked outside of New York to determine the allocation percentage.  But 1j excludes the days working "outside of New York" that were worked from home.  That is the application of the "convenience of the employer" test on the New York form.

And that form is used because New York will not allow businesses inside of New York to assume that their workers are not subject to New York taxation.  So, the NJ resident who is working for a New York company will have all of his income reported to New York.  If he took a one-month business trip to Texas, the portion of the income earned while he was conducting business in Texas will be excluded from his New York income, because he is not a New York resident and can't be taxed on income earned outside of New York by New York.  (New Jersey, of course, will be taxing him).  However, if he is telecommuting from home, New York does not allow those days to be excluded from the "total days worked within New York" calculation.

Hope this is helpful @20TCon.