The Configuration interface currently has no way to retrieve a list of items, such as might be represented by a TURF list or even a string with delimiters. See for comparison Apache Commons ImmutableConfiguration.getList(String key).
Implementing this feature will involve several steps:
Implement methods to retrieve a collection of objects, both as Collection<Object> and with a passed-in type.
Add a means to split strings (if so configured) in the string-based configurations.
Add convenience methods to get lists of other types, e.g. URI.
Update the key expression to retrieve indexed items in lists (if the collection happens to be a list), e.g. foo.bar.
The current thinking is to use Collection<> as the interface. We assume that configuration values are small in number, and there would thus be little reason to return something less rich such as Iterable<>. i.e. We should always expect to have the size. Stream<> is another option, but we can always get a stream from a collection, and the underlying value may be a Set<>, which we couldn't get back to (without collecting the values) if we returned a stream. Lastly we could return a List<>, but we might as well distinguish underlying implementations that are collections but may not be list (again without forcing item collection).
We'll continue the get/find dichotomy we have for the single values. That is, findObjects(key) would return an Optional<Collection<Object>>. This will allow us to distinguish between a defined empty list, for example, or an undefined configuration key, because a configuration key defined as  (an empty list) might be very different than undefined value which means we should use some default. Apache Commons Configuration considers a missing value as an empty list. Consumers who want that sort of functionality can simply use findObjects(key).orElse(Collections.emptyList()).
As with Apache Commons Configuration, we'll wrap single, non-collections objects in a collection of one. We'll probably use List<>, but maybe provide a facility to indicate the type of collection. (This could be used as variations of the normal retrieval methods as well.)