AJS Rich Text Editor (Rte) Shortcuts not working after confluence upgrade from AUI 7.x to 8.5.3

In AJS Version 7.x it was possible to deactivate a specific shortcut by executing
delete AJS.Rte.getEditor().shortcuts['ctrl,,,83'] → CTRL+S Quicksave

After the upgrade, this expression is “undefined”. I can’t find a suitable change of this function in the upgrade guide and documentation. Does anyone know what has changed here or if there is a better way to disable keyboard shortcuts (in this case the save shortcut)?