Go here:
https://support.microsoft.com/en-us/windows/windows-10-mobile-end-of-support-faq-8c2dd1cf-a571-00f0-0881-bb83926d05c5
Generally speaking, MS mobile support ended some time ago when security updates ceased. That's about the time I decided to switch although I'd been thinking about it for some time. I'd run into ongoing problems with vendors such as my bank dropping the WINDOWS
version of their phone app. I literally woke up and couldn't use an app to look at my account although I could still get to it through a browser. More particularly, MS is dropping their support for their own office apps this January. That doesn't mean that
the old app won't run on your phone right away, but you don't know when that happen.
They haven't given a day for closing the store to WIN10 mobile apps, but they haven't said they won't.
As for anti-theft, I guess the best the average person can do is to password their SIM and tie that to their startup protocol.
I'd start planning to switch sooner rather than later. I hated doing it, but I just got tired of watching my options shut down.