It will depend on the particular daycare. Some people are rigid about only allowing fixed dogs.
There isn’t a good reason to prevent a dog from signing up for daycare if it hasn’t been neutered or spayed. However, most centers will demand owners to disclose this information before booking a slot to avoid accidents.
Female dogs at a dog daycare who go into heat must be kept apart from the other dogs. Naturally, this will probably result in an additional charge.