[2021] Schlieffen commander removed, now requires retraining after patch.

This is correct.
Between Lighthouse and now, the ship was deemed "Special" and could have any commander put on it for free as if it were a premium. Now that it is out to the public, it is finally a pure tech-tree ship and requires specialized training for it.
This is no different than any other early-access ship from the past .
If you acquired it during the Lighthouse event, you should also have the default commander that came with it pre-trained, which I believe is 10 points.