Hi Jordan,
I believe that the only compatibility issue you could face was the change made to how polyscope uses the getTitle() and isDefined() method whic were introduced alongside polyscope 3.9/5.3, details of which can be found here. But providing that when you develop URCaps which use earlier APIs, you comply by the behavioural change, I believe you should be okay.
Bare in mind though that using the older APIs will of course limit the functionality available to your URCap.