My daughter has a 2023 GMC Sierra Diesel and it's at the dealer needing a coolant control valve, under warranty. However, apparently the part is backordered and won't be received and the truck repaired for several months. Why is this acceptable, and do prospective buyers know about stuff like this?

Apparently, the dealer is supplying a loaner vehicle at no charge...

 

Not acceptable IMO but at least they are stepping up and supplying a loaner. Maybe you could do some internet sleuthing and find a part quicker? Maybe even a wrecking yard; dealer may be happy to pop in a used part while wating for the new one if it's an easy swap.