Are they not building 2024 model year vehicles now?

If the 2024 model year has to end by 12/31/2024, that would be a pretty lengthy strike. The only time I ever heard of a model year being extended past September was the year the government required the next model year trucks to have tougher emission standards, and then the extended model year only applied to trucks. The model year that was extended to the end of the calendar year was 1977. The 1978 model truck production didn't start until Jan 1st, It was a very short production model year, and I believe the EPA was who enforced that. The EPA emission standards may now well determine the end date of the model year.

Past history shows that the model year typically ends on (or before) September 1st of the model year and just prior to that is when the production run would end, I'm not sure who determines the model year start and end these days. The production missed for any strike year has generally been shown as a reduction of the number of cars produced that model year.