While Microsoft has updated only selected Windows Phone 8.1 smartphones to Windows 10 Mobile, it has kept supporting WP8.1. But that is not going to be the case forever.
If you check the Windows Phone 8.1 life cycle support information at the official Page it seems Microsoft will stop supporting WP8.1 in July. The Q&A and lifecycle support page clearly mention end of mainstream support for Windows Phone 8.1 by July 11.
Some Windows Phone 8.1 users have today reported issues in signing into Microsoft account after a hard reset or after rolling back from Windows 10 Mobile.
Microsoft will continue its existing support for Windows Phone 8.1 until the end of support date of July 11, 2017. The Windows Phone 8.1 support date can be found here.