Accroding to Material Plugins | Babylon.js Documentation
// we need to mark the material
this.markAllAsDirty = material._dirtyCallbacks[BABYLON.Constants.MATERIAL_AllDirtyFlag];
Seems it has been replaced by Babylon.js/materialPluginBase.ts at 5.2.0 · BabylonJS/Babylon.js · GitHub
So the doc can remove this line safely?