Just another data point, it's contra to vancouver25k 's experience but in line with what rawilliam reported. Wanted to through check from AC to LH, separate tickets but both 014. The concierge says:

"You are correct in saying that we used to offer onward baggage tagging with Star Alliance Partner Airlines, however, this is no longer the case. Air Canada stopped offering this service, back in December 2023, for liability reasons. In order to tag checked luggage to the final destination, it must be on the same ticket."

I kinda get the liability reason, but not really. Every service AC offers incurs liability, e.g., letting elites check 3 bags for free rather than 1 increases AC's exposure to missing/delayed luggage claims.