How are configuration deprecations handled?

As documented in the “Breaking change policy” in CONTRIBUTING.md, features can be marked as deprecated at any point as long as there is a replacement available. Each deprecation is annotated in the API proto itself and explained in detail in the Envoy documentation.

For the first 3 months following deprecation, use of deprecated fields will result in a logged warning and incrementing the deprecated_feature_use counter. After that point, the field will be annotated as fatal-by-default and further use of the field will will be treated as invalid configuration unless runtime overrides are employed to re-enable use.