Essentially it's HFS Plus so Mavericks should have no difficulty restoring data that was backed up from El Capitan.
More generally, beware when considering a possible 'downgrade'; Apple is careful to avoid that word. For some apps, your user data from superior versions may be not entirely compatible with inferior versions. Don't assume that it will be possible to 'downgrade' any such data. You'll find many related discussions scattered across the web.