Update: September 22 2021, 07:58 PT
Element14 Community Raspberry Pi Senior Specialist, Phil Hutchinson has replied to our request for clarification. It seems that the customization service is no longer available.
“I’ve just checked we don’t currently offer a Pi Customisation service. We offered it up to Raspberry Pi 3. But customers are now using CM4 (Compute Module 4) or even the RP2040 so there is no longer demand, as commercially manufacturers weigh up the costs of small runs of custom Raspberry Pi Vs using CM4 or RP2040”