Under the tag "Change is constant", Cortana for (the now unsupported) Windows Phone 8.1 now declares that "By January this page will be retired, and I won't be using it to show you what's coming next." It does go on to say "I'll still be there to answer your questions and keep your reminders", so it appears that this is the effect of some old Microsoft service APIs being withdrawn at the end of 2018, i.e. next week as I write this.
Here's the screenshot proof today:
Read more: Cortana for Windows Phone 8.1 to lose some content next week