As per CHANGE-1525 the Priority will use avatar and not Icon. We don't have an endpoint to list all avatars for Priority. Only the system avatar will be listed by Get system avatars by type not the custom avatar. We can get an avatar for priority using Get priority but this needs a priority ID.

Suggestion:
Please provide an endpoint that list all the avatar used in the issue priority or include custom avatars in endpoint Get system avatars by type