The Wikipedia Mobile for iOS app relies upon srcset in its image widening routine. srcset should be reinstated for action=mobileview so the app and older versions of the app continue to function normally with image widening and generally functional gallery support (invoked from image tap). The mobile website HTML pages should continue to strip srcset, though.
Related patches identified by @Mholloway:
https://gerrit.wikimedia.org/r/#/c/270793
https://gerrit.wikimedia.org/r/#/c/286502/
https://gerrit.wikimedia.org/r/#/c/287818/
https://gerrit.wikimedia.org/r/#/c/287794/
The fix should be verified in the app.