ExtensibleObject relies on ClassToInstanceMap indirection through getExtensions() method. Since we have default methods (since Java 8), we can do better without the Guava tie-in:

interface ExtensibleObject {
    default <T extends E> @Nullable T extension(final Class<T> type) {
      // Iterate over availableExtension() and use isInstance() check
    }

    Collection<? extends E> supportedExtensions();
}

Given there typically are only one or two extensions, the search is equally-perfomant and users end up cleaner:

ExtensibleObject obj;
var ext = obj.extension(FooExtension.class);
// ... whatever

where previous they'd do:

ExtensibleObject obj;
var ext = obj.getExtensions().getInstance(FooExtension.class);
// ... whatever 

This will make it slightly ugly for extensions which build on top of each other, but we have exactly 0 of those and, at the end on the day, they'd have to co-operate anyway, so a reasonable implementation would use the same object – and we cover that use case.