They used as an indefinite subject (sense 2) is sometimes objected to on the grounds that it does not have an antecedent. Not every pronoun requires an antecedent, however. The indefinite they is used in all varieties of contexts and is standard.
They, their, them, themselves: English lacks a common-gender third person singular pronoun that can be used to refer to indefinite pronouns (such as everyone, anyone, someone). Writers and speakers have supplied this lack by using the plural pronouns.
The plural pronouns have also been put to use as pronouns of indefinite number to refer to singular nouns that stand for many persons.
The use of they, their, them, and themselves as pronouns of indefinite gender and indefinite number is well established in speech and writing, even in literary and formal contexts. In recent years, these pronouns have also been adopted by individuals whose gender identity is nonbinary, as illustrated in sense 3d above.