I am not a runner but I do like to go out for a slog when the mood occasionally takes me. However, my left knee feels like it has little cartilage left which is why I recently sold my singlespeed. What options do I have for “protective” running shoes?

Run on grass wearing whatever you want.

go to a running shop and get them to pop you onto the running machine analysis thing. They’ll see what you feet are doing then will be able to say what type of shoe you need. This will then help reduce the chances of further knee knackering

You don’t really have any options for protective shoes. Well, you have plenty of options for shoes, but any suggestion that any can protect knee joints is bollocks. Some may hinder things though.
You may get better protection by building up muscle strength, and you may want to measure what your running cadence is and considering increasing it if it’s on the low side.
http://runblogger.com/2015/03/effects-of-cadence-training-on-running-biomechanics-and-efficiency.html

cycling shoes (but not on a singlespeed).