It's not the software provider's duty to support every platform. Mullvad officially only supports Ubuntu, Debian and Fedora.

Their obligation ends there. By using any other distro, even a systemd one, you're taking responsibility to make it work in your system. That's the freedom that linux offers you! The ability to do whatever weird shit you want, at your risk and without any warranty explicit or implied.

Become a package maintainer for your distro to add support for mullvad and stop complaining.