You're right my friend. It can certainly be seen, later in the war, on 'second line' aircraft such as trainers and communications types, often on the fuselage spine, although it's use 'fell by the wayside' on most, if not all, front-line types, especially fighters.
Available photos from, for example, the BoB period, will show some aircraft with the patch, and many more without so, from a modeller's point of view, reference photos of the actual aircraft being modelled should be consulted. I admit to not adding the patch on virtually all models I've built from the BoB period, and not at all after late 1940.
However, the use of Gas Detection patches, strips and cards, was still standard on military installation buildings and other permanent structures and so on, and all RAF Stations, for example, had a Gas Precautions schedule and a small staff to administer the monitoring, throughout the war, with the carrying of gas masks being mandatory.