Microsoft has confirmed that support for the original Windows 8.1 software will be discontinued in favor of the more recently released Windows 8.1 Update. They will be treating the Update version as it's own service and support baseline, and users who wish to continue to receive security patches will have to migrate to Windows 8.1 Update.
If you opted to manually download and install updates in the original Windows 8.1 software, you'll have 30 days in which to install the Update version. After those 30 days are over, any newly-downloaded updates will be marked as not applicable for your device.
The source link has a full list of related tech bulletins and articles for those who want to dig a little deeper into the "why this was done" question. For the rest of us, just know that it's time to install Windows 8.1 Update if we want to continue to participate in Patch Tuesday.