MediaWiki has an old system for responding to AJAX requests that predates the action API. This system is implemented in the AjaxDispatcher class and controlled by the AjaxExportList setting. It was kept around to cover use cases that didn't fit well within the framework of the action API. However, these use cases can now be covered nicely with the new REST API, so the old AJAX interface should go away.
Steps:
[] soft-deprecate the legacy mechanism (update documentation)
[] remove usages in MWF extensions: https://codesearch.wmcloud.org/deployed/?q=AjaxExportList&i=nope&files=&excludeFiles=&repos=
[] survey usages in other extensions:
The first step is to officially deprecate it, convert current use cases to REST endpoints, and then cause the old mechanism to emit deprecation warnings.
[] hard-deprecate AjaxExportList usage
[] remove AjaxDispatcher and erelated classes