On the Car Designs window, the “Delete This Model” button is always greyed out.
I understand it’s probably not fully implemented yet to avoid having cars in construction being “orphaned” and blocking the queue.
My suggestion is to change the functionality from “delete” to “retire”.
The retired designs can no longer be edited, no new chassis can be produced (automatically removed from the order list of the first slots) but those remaining on the production line could still be produced to the specs.
Yup this is exactly the plan, the button will eventually just hide the design from that list (and as an option in production schedules). I just wanted to get the latest patch out before I had time to implement this, but it will get done this week.