https://www.bleepingcomputer.com/news/security/microsoft-now-lets-you-enable-the-windows-app-installer-again-heres-how/

Microsoft now allows enterprise admins to re-enable the MSIX ms-appinstaller protocol handler disabled after Emotet abused it to deliver malicious Windows App Installer packages.

App Installer (also known as AppX Installer) allows users to install Windows applications directly from a web server using an MSIX package or App Installer file without first downloading the installers to their computer.