Two-way sync: very nice!
Yup… They first announced they were shutting it down in April 2017. To their credit they’re not leaving anyone high and dry, and are building up Microsoft To-Do in the mean time (while also giving users plenty of time to switch to it, or to something else).
The link I posted is to the web login. It works just as well as the iOS app so you might find that the web app is good enough. (If not there are plenty of alternatives.)
FYI you might find this tips&tricks article for the app to be useful: