Currently users have to choose to open all pages from the *.wikipedia.org domains in app or in browser on iOS. This means that some special pages not available in the app result in an error message, and the user has to go back (best case) or delete the app (worst case).
We already have tickets in to allow users to explicitly choose to use deeplinks or not (if the OS can support it). However, this would still be valid in that case, so that the user has even more fine grained choice between app and web when they need it (as in Special and Flow based pages).
See community question/comment/request here: