KML content isn't available in Google Maps

While using Google Maps, you might have received a notification saying "Some custom on-map content could not be displayed."

That’s because a certain type of content isn’t available anymore in the classic version of Google Maps or in maps embedded on other websites from the classic version of Maps. The content type that’s no longer available is KML, the file format Google Earth uses for the exchange of geographic information.

If you're the owner of a custom map with KML content, you need to update your content. Or, you can import your KML content into My Maps.