Chaff is broken. FD have acknowledged the bug and said that they are investigating. It is inconsistent across ships - NPC Anacondas with turrets are immune to chaff whether you have targeted them directly or not. Targetting a ship was never previously a requirement for chaff to work on it.
Chaff no longer delays security scans. Many of the Stockholm Syndrome crew on the official forums insist that it was never supposed to stop scans but before 1.4 it did. Now it doesn't.
You can tell when your chaff has worked against a gimbal or turret because the beam/bullets will wave about wildly although they can still hit you if they happen to wave across your ship. Since the bug appeared you can tell when chaff doesn't work because when you pop it the NPC's beams/bullets don't budge at all. When an Anaconda is sideways-on to you and there are four laser beams from it trained directly on your ship you know it's not using unchaffable fixed weapons.
Unlike most of the people who claim that chaff is not broken, I do use chaff all the time. But the fact that FD have acknowledged the bug should be an end to people saying that chaff is not broken:
https://forums.frontier.co.uk/showthread.php?t=193600