And that would have been done since 2001 prototype. But the real question is whether any flight without LL has been outside of China's airspace where they would increase the risk of exposing its radar, electronic, and infrared signatures to anyone who's got the equipment to gather them (US and Japan).
You realise it’s not just having the equipment that matters, but also being within range right?
Stealth aircraft are really only effective against short frequency fire control radars, and China has probably the world’s biggest (in variety and number of operational units deployed) arsenal of dedicated low frequency anti stealth radars in the world.
While such radars might not have the accuracy to give firing solutions, I think it would be pretty unrealistic to think enemy stealth aircraft can get within a couple hundred miles of the Chinese mainland coast without having to seriously worry about being detected and exposing their own signatures.
As such, I think the likes of the J20 would be able to operate at least within 200km off the Chinese coast over international airspace (as obviously overflights of Taiwan and other non-Chinese land features would be a bad idea) in clean configurations without needing to seriously worry about anything being able to get remotely close enough to even detect them, never mind register anything useful signature wise.
Normally J20s operating with LLs were primarily to help air traffic control since they were operating near major Chinese cities with a lot of civilian air traffic.
The lenses were probably also an added safety/security feature so that the J20s could be tracked at all times so that first responders can get on the scene ASAP should one go down, since even fragments of the plane could yield invaluable intelligence to hostile countries, much like like pieces of the downed F117s over Kosovo were early snapped up by Chinese and Russian intelligence agencies.
As the J20 matures, such concerns are probably receding.
