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 $wgAjaxExportList setting.
$wgAjaxExportList has been deprecated since 1.27. 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.
- remove usages in WMF extensions: https://codesearch.wmcloud.org/deployed/?q=AjaxExportList&i=nope&files=&excludeFiles=&repos=
- survey usages in other extensions: https://codesearch.wmcloud.org/extensions/?q=AjaxExportList&i=nope&files=&excludeFiles=&repos=
- hard-deprecate AjaxExportList usage
- remove AjaxDispatcher and related classes