The incremental xDS design has been decided on, and added to the appropriate .proto file (in #3470). Now, incremental xDS clients should be implemented in Envoy. I am currently getting started on CDS.

Relevant Links: an early draft proposal for incremental xDS, still relevant for examples and overview of goals: https://docs.google.com/document/d/1dh8ACRzNNUkVYB3exepTPlLQK6CyDrOaP2OwwGlxDB4/edit#heading=h.yfahbzhf2783