Is 401k employer match included in W2 Box 1?
No. The employer match is not included for several reasons.
The vesting schedule is different for each company, so that they would have to tell the IRS that the match was X$'s this year for this employee, but the amount that was vested is Y. The next year some of the first years money would have to be declared by the company as new covered by vesting.
The bigger issue is that no matter if the employee contribution is pre-tax, post-tax or Roth, the match has not been taxed and will be taxed when the money is withdrawn either during retirement or early.
Check that the amount stated on the W-2 matches what was on your last pay stub for the year in question.