As I understand it, the legacy camera support is for allowing the original Raspberry Pi Camera Module to be accessed via v4l2, which gstreamer makes use of through v4l2src. Newer camera modules (2/3) are not supported by that, and need to be accessed via libcamera (libcamerasrc in gstreamer), which our default gstreamer configuration does not make use of (hence @joaoantoniocardoso suggesting you try to set that up separately).
It is currently expected that new Raspberry Pi camera modules will not work until support has been added for them.