ExtensionContext.registerSubsystem has an overloaded variant that takes a boolean 'deprecated' parameter. We never use it.
For deprecated subsystems we perhaps should.
I think most of our deprecated subsystems are using the SimpleResourceDefinition.setDeprecated API to trigger deprecation logging if the subsystem is actually added. So this extension call really is just for the case where the extension is included in the config but is not used for adding a subsystem. That's kind of a corner case so it's a valid question whether we want to add this setting and end up writing two messages to the log.
So this issue is to track at least considering it.