Multiplatform Runtime only works with the Extended Maintenance versions of Vaadin 7 and Vaadin 8.
Multiplatform Runtime (MPR) makes it possible to run applications and components written with Vaadin 7/8 inside a Vaadin Flow application.
Issues can be reported on the public Multiplatform Runtime repository.
Follow the Step-by-Step Guide to configure your project to use MPR and migrate your legacy application to a newer version.
Configuration and advanced topics
Using Legacy Components In a Flow Application