My Debian server has a mix of systemd and /etc/init.d startup scripts. That’s the sort of thing where a BSD would be likely to say, ok, as of version N due in 3 months, we’re migrating everything and going from 100% old way to 100% new way.
You'll be happy to know that this is going to happen. Quoting https://github.com/systemd/systemd/blob/v255/NEWS
> Support for System V service scripts is now deprecated and will be removed in a future release. Please make sure to update your software now to include a native systemd unit file instead of a legacy System V script to retain compatibility with future systemd releases.