Is the Grand Canyon accessible sitting the government shutdown?

Yes

Yes. We visited Grand Canyon Park two days ago. The entrance is open and free. All parking lots are open. Local visitor center is closed but gift store is working. Toilet rooms are working. There is a shuttle inside the Grand Canyon Park to connect a few parking lots.

https://arizonadailyindependent.com/2018/12/25/grand-canyon-national-park-accessible-to-public-during-partial-government-shutdown/

I believe it is because there is so much self-generated revenue from the park. I know I read somewhere about it being one of the bigger parks that are still opened, there just may be less services offered currently because it will mostly affect payrolls on the bottom levels.