RIP QuickTime for Windows. Apple is "deprecating support" for the application, and will no longer patch security flaws in the software.
In this regard, QuickTime for Windows now joins Microsoft Windows XP and Oracle Java 6 as software that is no longer being updated to fix vulnerabilities, and subject to ever-increasing risk as more and more unpatched vulnerabilities are found affecting it.
So, Windows users should uninstall Quicktime. How to do so is detailed here:
https://support.apple.com/en-us/HT205771
Mac OS users should not do so as the security risks that have prompted this do not affect the Apple OS.