Not sure if this has been asked before, but since this forum has not a good search.

The 'old' firmware had 3D Noise reduction, which is now gone in the newer firmware. This 3D noise reduction seem to have given problems in normal lighting conditions for things like trees and grass, and... in my case, the ocean.

Is this 3D noise reduction now default 'on'??
Hope not, because my latest video show on highest resolution still some 'blurriness' flying over water. Reduction from 4K -> 1080p should give a sharper image usually, but in my case, the sea get 'muddy' . I shoot with D-Cinelike, but I'm afraid that this noise reduction together with the 'blowup of dark parts' will create this muddy blockiness images .... would be irritating since D-Log on the Phantom sucks even worse, so... looks like these profiles are a bit useless ... (PS is D-Cinelike also raising the ISO like D-Log does??)

Some advice on youtube states to put this 3D Noise reduction off... but since it's not an option in the menu anymore... is it on all the time??

Very good question, we need an answer

3D Noise reduction is for when you have your video camera sitting on a tripod with little movement in the image.  It should normally be off on a drone.

3D noise reduction is mainly used for low light, but... what it does/did in 'normal' video is best explained here: