As part of #201579, we added some special code to handle the case where enabled is provided in the config but it's not available in the schema.

Since that's a specific piece of code, we can now control the error message. Should we work out a more human-friendly error message or are we happy with the current joi-generated error message:

[config validation of [myPlugin].enabled]: definition for this key is missing

With that new piece of logic, we could change that message to something like:

"myPlugin"'s enabled status cannot be changed. Please, remove "myPlugin.enabled" from the configuration file.