Catalina printer disappeared

Yes, but that doesn't mean that vendor drivers aren't supported; Software Update is downloading/installing vendor drivers, and I think this is to make sure that the most recent drivers are installed - not the old drivers that came with the printer.

John Galt wrote:

Catalina supports only AirPrint-enabled devices. Remove the existing printer queue in Printers & Scanners and add it again, this time being sure to specify AirPrint.

Using AirPrint for this model is the right approach, but Catalina does support non-AirPrint printers, if the vendors have updated their drivers to 64 bit.

Excerpted from Add a printer on Mac:

"Don’t install the software that came with the printer or from the manufacturer’s website; follow the instructions in one of the tasks below, and macOS will prompt you to download the latest software if it’s needed."

Catalina supports only AirPrint-enabled devices. Remove the existing printer queue in Printers & Scanners and add it again, this time being sure to specify AirPrint.

What model Pixma is it? Are you using USB or wireless?