From the discussion in this av1-avif issue and the last SG-Image meeting, the conclusion was that the spec now has everything needed for doing progressive refinement either via a separate thumbnail image or via multi-layer coding.

It was concluded in the meeting that the next step would be for implementations and tooling to start using it. Adding a downscaled, very low quality thumbnail to images would be a good step as shown in @jakearchibald's experiments in the issue linked above. @skal65535 mentioned that avifenc currently does not support adding a thumbnail, or at least not to add an automatically downscaled thumb with low quality. Adding that functionality would be a good way for people to start generating content that can then be used to test decode implementations.