Ryubing emulator was removed from flathub. Help me!

Well, as it was already mentioned, the application had to be removed due to a DMCA Takedown. So the application has been set to be EOL (end of life).

Now, Flatpak itself does not remove applications that are EOL. KDE Discover however does strongly encourage to remove unmaintained applications. That’s the prompt you have seen.

As for reinstalling it: If it were a simple EOL, you could AFAIK still use the flatpak install command to install it, if you know the application id, as the application is only hidden. That will probably not work though in this case, as when an application was removed due to an DMCA takedown, the application has to be removed from the servers.