I'm not sure why your client got that notice, but if it is a straightforward as you say, here is the text of the TCJA as signed into law that answers it:
Sec 11(b) of the tax code now reads as follows:
PSC C corps now pay the same rate of 21%. Some early proposals had the PSC rate at 25% but that didn't make it to the final legislation that was passed.
PHC tax and accumulated earnings tax are both still in effect, rate unchanged at 20%.