Since iOS v.5 was released (? I think), edits tagged as "mobile app edit" dropped massively; I'm assuming that this is because we started asking people to opt-in to EventLogging/etc. (good!) and that we applied it to non-tracking, though looking at the code it doesn't look obvious how that would be the case…
So, we had a pageviews issue with 5.0 where the user agent changed and the EventLogging tagging was missing iOS. We fixed that on both the EventLogging and client app sides pretty quickly, though and I believe also re-generated missed data. Not likely to be the exact same issue here, but potential area of investigations.
The intention in the opt out was not to opt out of normal editing related tracking (ie. IP adress for anons, activity by logged in editors, etc), so if that has been the effect we should fix.
As for a "organic" drop, it is possible, given that we no longer pop up a log in on first install, but I would seriously doubt that would change this so drastically. We also changed the editing icon slightly, but it is as proment as ever, and the editing flow itself was not changed at all in 5.0
The actual issue we noticed appears to be with the Android app: T141667: Android app does not set the "mobile edit" change tag