Link to unofficial flatpak. Please contact the maintainer of this flatpak and tell them you want to take control of this flatpak and make it verified and official. That way the flatpak will also be kept up to date since it'll be maintained officially.

Benefits of distributing as Flatpak

• Shows up in app stores of user-facing Linux distros (flatpak has been adopted by all of them except for Ubuntu) OOTB making it easier to install and discover on those distros.
• Flatpak is updated with the rest of the system from the software center and can be set to automatically update alongside it as well.
• Flatpaks are sandboxed (I dunno that if this will be of any benefit to Simplenote because I am not a security expert).

I would even recommend to retire the RPM and the Appimage to lessen the maintainance burden by reducing the amount of distribution methods and instead replace it with the flatpak since it can be easily installed if it isn't already installed on any distro but keep the DEB for Ubuntu because it doesn't have flatpak installed OOTB and its update center doesn't support updating flatpaks once they are installed.