Systemd defaults to the kernel's standard way of unmounting filesystems (i.e., umount(2)). You would have to write a .mount file only if your filesystem _couldn't_ be unmounted the normal way.

The alternative for systemd would've been to shutdown the system without waiting for a clean unmount, which can lose data, which is what used to happen with SysV. The alternative for you would be to unmount your filesystem yourself before shutdown, which is what people used to do with SysV. Some SysV distros even had scripts that tried to automatically clean up mounts before shutdown, which is exactly what systemd does now, and your 90-second hangs would've shown up there too.

> It leaks into user land.

Init has always been in user land.