When you visit websites that host Flash content, Internet Explorer 11 will warn you that Adobe Flash will not be supported after December 2020.
In July 2017, Adobe made a joint announcement with Apple, the Facebook, Google, Microsoft and Mozilla that Adobe Flash will cease to be supported by all browsers.
With its current release optional cumulative information of Windows 10 for versions of Windows 10 1909, 1903 and 1809, Microsoft warns users that the end of Adobe Flash is coming soon.

For those who do not install the current optional updates, this change will be included in its updates Patch Tuesday of October.
By installing this update, Internet Explorer will display a warning to websites using Adobe Flash content that Flash will not be supported after December 2020.

This notification will be displayed to a user once every 60 days until December 31, 2020, for each site you visit and host Adobe Flash content.
For those who do not want to see this notice in Internet Explorer, you can configure the value "DisableFlashNotificationPolicy" in the section HKEY_LOCAL_MACHINE \ SOFTWARE \ Microsoft \ Internet Explorer \ Main, as shown below.

Microsoft plans for Adobe Flash
Clicking the "Learn more" button in the alert will take you to a page that explains how Adobe Flash support is stopped.
"Microsoft terminates support for Adobe Flash Player on Microsoft Edge (both the new Microsoft Edge and Microsoft Edge Legacy) and Internet Explorer 11 at the end of 2020 ″, Microsoft explains.
After December 2020, Adobe Flash Player will no longer receive security updates and all publications which are older than the one released on June 9, 2020, will be automatically turned off at Browser.
In the fall of 2020, an optional update will be available entitled "Update for Removal of Adobe Flash Player" which will remove Adobe Flash from Windows. This update is permanent and cannot be uninstalled.
Finally, in the summer of 2021, Microsoft will release a mandatory cumulative update that will permanently remove Flash Player and all related API.