You can still use a keyscript (I do), it just need to be done in a different way. Instead of putting the configuration in /etc/crypttab, which confuses systemd indeed, you can still use the kernel cryptopts variable. For example, in the grub configuration add something like:

cryptopts=source=/dev/<disk>,target=sdc5_crypt,keyscript=/lib/cryptsetup/scripts/passdev,key=/dev/<other_disk>:/somdir/root.key

Anf then in /etc/fstab use the /dev/mapper/<target> (sdc5_crypt in the example above) as the root device, as before.

Then you need to make sure the initramfs contains all the tools needed to support this (that was done automatically with /etc/crypttab, it's "manual" with the kernel option). To do this, add under /etc/initramfs-tools/hooks a script file to load what's needed in the initramfs: cryptsetup, passdev, the needed kernel module. You can roughly copy the existing /usr/share/initramfs-tools/hooks/cryptroot and simplify it.

I've seen other distro documenting the kernel approach instead of /etc/crypttab for the root filesystem. It may become the standard way in the future, and there's no reason it couldn't be fully automated. There's some coordination between several components so it may take a bit of time to converge to an accepted and supported way thought.