What is the latest AmigaOS for the 500?

[countzero]

if you have kickstart <= 1.3 => workbench 1.3
if you have kickstart <= 2.04 => workbench 2.1.

PS : I don't think any stock A500 shipped with kickstart 3.1. If you have 3.1 ROM you can use workbench 3.1 of course. Actually you can run wb 3.1 with 2.04 kickstart too, but you have to install it somewhere else in a kick 3.1 environment, then connect the HDD to the 2.04 machine.

[countzero]

there's no 020 code in 3.1, you can run it (I run it) on a stock A500/A600. You need the 3.1 ROMs. Amigakit or Vesalia should have 3.1 ROMs for A500/600/2000. Failing that, check ebay.