Why does my car keep knocking and pinging after I turn the ignition off?

It's called dieseling, https://en.wikipedia.org/wiki/Dieseling - try cleaning spark plug of carbon deposits or a good fuel injector cleaner, and check for trouble codes, esp. too rich conditions. You shouldn't need to switch to higher octane fuels as a solution, if car is clean and maintained it should run on regular unleaded just fine.

check your oil and the level of it, sometimes its the oil that could cause the noise amongst other things but that is where i would start. good luck ty

Can't imagine you don't have a CEL accompanying this nonsense. Likewise, Subie went to iridium plugs by now, so they should NOT be fouling. Nonetheless, ensure that the proper heat range plugs are in there and follow any CEL trail. Maybe there's a vacuum leak causing enrichment, especially at idle speed? So use your ears too!