On May 13, 2024, the types of these changed. Before, they were public string[]; plain string arrays. This put the burden of proper encoding on the user, increasing the probability of bugs. Now, they are RecipientLists - internally, an array of EmailRecipient objects, but with a wrapper to provide compatibility with the old string-based api.
Recipients of the message. You can use operator ~= to add people to this list, or you can also use addRecipient to achieve the same result.