Thanks, PAHU, for providing the Australian site for the Canon driver. I have an MP500 and it still seemed to print fine after I downloaded Yosemite. However, as others have reported, the MP Navigator no longer performed normally in scanning. The "Save As" keys didn't align and therefore didn't work but one could still scan using an alternative method ("Scan with scanner driver" button).
In any case, my hard drive subsequently failed on this 2010 27" iMac and when it was replaced the tech installed Yosemite 10.10.1. Just tried printing this evening and it wouldn't. Advised I needed undefined "software." Got various error messages. Went to Google and read this and downloaded that to no avail. Tried the Gutenprint driver but it too didn't work. Latest error was "stopping job because the scheduler could not execute a filter." Had no clue what that meant! System info listing for the printer (under About this Mac, System Info) lacked any reference to permissions or a printer utility version (but I didn't notice or know significance of omissions). Finally I downloaded from the Australian site, just the print driver 10.51.2. The printer then printed!!! (The MP Navigator scanner remains flawed but works by clicking "Scan with scanner driver" rather than the normal way.) I'm holding my breath that things will continue to work.
Thanks again, PAHU, for your help, and Greg and Pam and Andy too!