A lot of debian packages supply both systemd unit files and init scripts. You can use either to step on the other's foot.

As I said, init scripts are only used if there's no unit file. If there's a systemd unit, the init script is ignored.

If there's a systemd unit, the init script is redundant, confusing, and possibly dangerous. It should not exist in the first place, perhaps except as a thin wrapper around the corresponding systemd command.

If you invoke the init script on a system booted with systems then it does indeed wrap the equivalent systemctl command.

If the package maintainer wants to support systemd-less systems out there, I don't see a reason to not let them to.

Having that support come in the form of installing both on all systems does have its downsides though.

OTOH, I wouldn't be surprised if every other feasible approach's downsides are widely regarded to be worse.

Software. Yay :D