Difference with what is now in core?

It does look rather similar, doesn't it? I wonder if this is why the Drupal 11 compatibility for this module hasn't been progressed - it's just not needed any more.

I too would love to know if this module supplies anything the new core features otherwise miss.

Looking at this again, as I realise this module isn't yet D11 compatible. Does it need marking as unneeded, or does it need updating to work with D11?

“This will only display if form elements are correctly setup with #config_target keys, and if that target is overridden.”

This module can be useful until the mentioned render array attribute gets widely used in the contrib space. Otherwise, the core's warning will not appear if #config_target is not communicated by the ConfigForm instance.

Yes, that makes sense, so I guess a D11 version is going to be important

This module can be useful until the mentioned render array attribute gets widely used in the contrib space.

Yes.
In addition: this module also shows overrides for config entities. (It's perfectly possible to override config entities' properties using $config in settings.php.)

We have somewhat simple entities, in a form that extends EntityForm, and this module correctly reports (I changed the names):

In the config file MODULE-NAME.ENTITY-TYPE.ENTITY-ID
- The value for PROPERTY-NAME has been overridden.

I don't see a plan to support this in Core. (And I don't know enough about overriding config entities, to know if changing the code to support this, would be simple. But at this moment, all the code is in ConfigFormBase, which != EntityForm.)