Microsoft stops providing security updates to PCs with no Anti-Virus software
support.microsoft.com
support.microsoft.com
There's a compatibility problem between some AV products and Windows, so unless you jump through hoops, they won't deploy security updates to systems that don't have any such products installed? How does that make any sense at all?
seems more like Microsoft stops proving security updates to PCs with Anti virus that breaks Windows after the update
Edit: Also looks like the no anti virus part is just to allow anyone who don't run compatible anti virus / no anti virus(which sets the registry key) to force the update
Their "fix" also affects those that don't run any AV software at all, because the registry key won't be added automatically. Unless you are aware of the issue and add the key manually, you won't receive any future security updates until such time as Microsoft stops checking for it. Currently incompatible AV software can be automatically patched in the future, non-existent AV software can not. Hence the title.
The sensible move on their part would have been to add the key if no AV software is registered with the OS, instead of just leaving all those that don't run AV out to fry until they become aware of the issue and fix it themselves.