Loss of shower pressure.

While recently showering, my water pressure suddenly dropped about 50%. There was no change in temperature, and the pressure in the rest of the house is fine. It was time to replace the shower head anyway, which I did, but the (lack of) pressure remains. I'm on city water.

It is not uncommon for debris to clog a shower mixing valve but that would create a temp change which you say did not happen. I would still remove the mixer, clean it, flush the valve by trunng on the water for a second with a towel held tightly over the mixer opening.