After two hours with NY, I have an their response to this situation. I thought I'd post here in case someone else runs into this problem.

IT-222, the form on which you calculate the NY individual GCT credit for taxes paid as an S-Corp Shareholder, was not updated for 2020. NY's instructions are to use the 2019 form instead, since the credit is still valid and nothing on the form changed for 2020.

Well, the 2019 IT-222 form is not in ProConnect at all, so you need to download the form and calculate the credit outside of the return (which seems like a PTO miss), but that's not the biggest problem.

The 2019 IT-222 form says to place the credit on form IT-201-ATT on line 8a. Turns out, the 2020 IT-201-ATT removed line 8a (which seems like a NY testing miss, but... pandemic?).

Their solution is to prepare the NY return on paper and mail it in, writing the credit into line 8a and (I supposed) manually carrying it through the rest of the return.

If anyone has a better idea for how to shoehorn a completely valid credit into the NY return, I'm all ears. The suggestion I got from help/chat was to force it onto line 8, which is the line to enter the UBT credit.

Thanks!