If I am reading this correctly, the shelter has not requested expired carseats, but this FB group is encouraging people to donate any carseats even if they are expired. More than likely, a shelter will not appreciate that at all! Then they will have to check all the expiration dates and trash the ones that are unusable. It is the same rule as for clothing. You just do not donate your trash. If it belongs with the trash, then discard it.
Also, unless the shelter has a lot of kids needing carseats immediately, then they will have to discard or donate some of them elsewhere. The shelter probably does not have space to store a lot of carseats. They have space to store food, diapers, food and formula, probably some clothing and blankets, but not much else. I have worked for a women's domestic violence shelter before and we definitely did not have the extra space to store a lot of carseats. But, we did allocate money to purchase carseats as needed.
It was always extremely obnoxious when people would drop off clothing donations that even Goodwill would have trashed. Rule is if you could not sell at a consignment sale, do not donate it!