From Twitter, users of Cortana on iPhones and Android handsets have started getting warnings, for example:
And from Microsoft's official support document:
As we announced in July, we will soon be ending support for the Cortana app on Android and iOS, as Cortana continues its evolution as a productivity assistant.
As of March 31, 2021, the Cortana content you created–such as reminders and lists–will no longer function in the Cortana mobile app, but can still be accessed through Cortana in Windows. Also, Cortana reminders, lists, and tasks are automatically synced to the Microsoft To Do app, which you can download to your phone for free.
After March 31, 2021, the Cortana mobile app on your phone will no longer be supported.
As suggested, Cortana-created content is still in 'To Do', which is completely cross-platform and current (even on Windows 10 Mobile), plus Cortana as a voice assistant still exists on Windows 10 Desktop, albeit being in the process of getting sidelined in Microsoft's platform roadmap.
Apple's Siri and Google's 'Assistant' are now dominant on phones, with 'Alexa' in the home. So there was room for a third voice assistant in the market, you see. Just not on smartphones!