Microsoft is testing the removal of Adobe Flash, that is scheduled for December 2020. By that time it will be discontinued in the majority of web browsers. KB4577586 allows you in advance to see right now what will happen.
The patch can be downloaded from the Windows Update catalog. It is titled as Update for Removal of Adobe Flash Player for Windows <version here>, and is also available for Windows Server editions.
After performing the update, the Flash Player can no longer be installed. As Microsoft describes it, it would then only work with a system restore point or a backup created.
The update is only available in the Windows Update catalog, i.e. it has to be downloaded manually. It is not available via the built-in Windows Update service in Windows, so causal users can't accidentally install it.
The idea behind this update is to allow Administrators to ensure that the system can work properly without the Adobe Flash component.
If Microsoft didn't change its plan regarding the Flash removal, it will be discontinued in the Edge Beta on December 7th, 2020, and in the Stable version on January 21st, 2021.
Adobe will stop distributing and updating Flash Player after December 31, 2020.
Adobe Flash is now the only NPAPI plugin that Firefox supports. Starting in version 84, Mozilla will remove all NPAPI code from the browser which is required to run Flash. Chromium-based browsers will also drop Flash support starting with Chromium version 88 which is coming on January 2021.