24 March 2021
UPDATE (August 3rd, 2020):
Microsoft has now decided to shut down the Cortana Android and iOS app around the globe. The support will be ended by early 2021 which is towards the beginning of the next year. This is essentially the final nail in the coffin for Cortana as long as availability for general consumers is concerned. The assistant will continue to be available through Microsoft's productivity tools.
Original (November 18th, 2019)
About four years ago, Microsoft released a dedicated Cortana digital assistant application for Android and iOS users. The aim of the release was to expand its user base as it looked to compete with Google Assistant and Apple's Siri. It seems like Microsoft has realized that this is a battle it cannot win and has decided to kill the application starting next year.
Microsoft has confirmed that the Cortana app for Android and iOS will not be live after January 31st, 2020. It will kill the Play Store and App Store listings for the app in the United Kingdom, Canada, Australia, India, Mexico, China, Germany, and Spain. It has said that the app will continue to be available in the United States but has not revealed for how long and in what capacity.
It will also release a Microsoft Launcher update that will remove the Cortana integration it added last year. The official announcement mentions that Cortana will be added to Microsoft 365 suite of productivity apps "to make your personal digital assistant as helpful as possible".
If you use Cortana on Android and/or iOS for any type of content creation, all of that will still be accessible via Cortana on Windows. As for your reminders, lists, and tasks created through the digital assistant, they are synced with Microsoft's To Do app. It is a to-dol ist app that can be downloaded on one's Android and iOS devices for free.