Backward Compatibility
The property that a new version of a producer continues to work with clients written against the old version.
The discipline that makes independent deployment possible. Without it, a service and its callers must be released together, which is the coupling microservices were adopted to remove.
The rules are mechanical: adding an optional field is safe; removing a field, renaming one, narrowing a type, adding a required field, or changing the meaning of an existing value are not. The same rules apply to events on a queue, where they matter more, because a consumer may read a message written months ago.
The hard part is not knowing the rules but knowing when they are broken. Contract tests or a schema registry with a compatibility mode configured turn "we should be careful" into a build failure.