Reader View is unfortunately no longer being maintained since Mozilla decided to integrate Pocket. It bugs me since I find Firefox's Reader View actually does a better job than Pocket in most cases of stripping out the clutter.
Pocket were added as it were easier than implementing their own Reading List, although Pocket will be shipped as a default extension in the future rather than being integrated. https://bugzilla.mozilla.org/show_bug.cgi?id=1215694
You sure you're not confusing Reading List and Reader View? Work on Reader View is still happening in bugs such as this one: https://bugzilla.mozilla.org/show_bug.cgi?id=1175536