Does the letter start.....  Effective September 3, 2019, the procedure for electronically .......

If so, it is a form letter sent to all SCorps and LLCs. 

Yes.  Does that mean my returns do not need to be amended?

Only have to amend if you did what is described in the letter.      Had pass-through withholding credits and attached IN-K-1 rather than entering into the appropriate schedule. 

The withholding credits would be for non-resident shareholders/members.   Did any of your SCorps or Partnerships have non-resident shareholders or members?