Here's Cortana's new existence, as a 'personal productivity assistant' for Office 365 users (graphic from Microsoft):
In terms of the sort of things you'd ask it, there's little change from Cortana of old, except that this 'experience' isn't now baked into the platform itself and not available to most people - you 'd have to be signed in with a Microsoft 365 account (like Office 365 for consumers, but including some extras and aimed at Windows 10 for enterprise users). It's also not available outside of the USA initially...
From the Microsoft article, which is desperately trying to put a positive spin on the changes(!):
AI in Microsoft 365 is driving a significant shift in how people interact with Microsoft 365 applications. Our experiences personalize and adapt to you, support you and help amplify your skills. You can see this shift in the personalized experiences we are enabling through Cortana, your personal productivity assistant in Microsoft 365. Cortana helps you stay on top of your day, save time and do your best work.
Today, Microsoft is announcing an updated Cortana experience in Windows 10 that will deliver more help from your assistant in Microsoft 365. This next step in Cortana’s evolution will bring enhanced, seamless personal productivity assistance as a free update to the latest version of Windows 10 coming this spring.
Through this updated Cortana experience, we will roll out new Cortana services delivered through Microsoft 365 backed by the privacy, security and compliance promises of Office 365 services as set out in the Online Services Terms. Protecting your data and privacy is our highest priority, and we give you control over your data.
The upcoming update to Windows 10 will include access to a new Cortana experience with an emphasis on productivity, helping you quickly find the information you want across Microsoft 365. The new Cortana experience in Windows 10 features a chat-based UI that gives you the ability to interact with Cortana using your voice or the keyboard.
For English (United States) users, Cortana will assist you in better managing your schedule and tasks by staying on top of your calendar and focusing on what matters with meeting insights. You can speak or type requests to find people or files, or quickly create or query emails. You can also easily check your calendar, set a reminder, or add to your lists in Microsoft To Do:
“What’s next on my calendar?”, “Remind me to send the ‘weekly report’ every Friday at 2pm.” “Add ‘status report’ to my task list.”
The article then goes on to mention 'older' versions of Windows, including Windows 10 Mobile:
We’re also making some changes to where Cortana helps you. As part of our standard practice, we are ending support for Cortana in older versions of Windows that have reached their end-of-service dates. We’ll also be turning off the Cortana services in the Microsoft Launcher on Android by the end of April.
We’re excited about how these updates to Cortana will help you stay on top of things, save time and do your best work. As we continue to innovate on Cortana in Microsoft 365, we plan to share further improvements in the coming months.
Windows 10 Mobile is, of course, an 'older' version of Windows and so Cortana support is ended. As you've probably noticed. The Cortana search tool in the toolbar of the UI will contiunue to bring up a place to type queries and - in theory - will pass these on to Bing. Or just use Bing searching in Edge in the first place.
The withdrawal of Cortana on Android is a bit of a shock - I'm assuming that Google Assistant will then be front and centre on the upcoming Surface Duo. Microsoft, Samsung and Google really are cosying up these days, eh?(!)