grammar - Anyone: ("they" or "he/she") why is it sometimes plural?

Plural versus singular:

Anyone can learn to dance if they want to.

Anyone can learn to dance if he or she wants to.

Resources online tell me that anyone is a singular indefinite pronoun. Then why is it sometimes acceptable to use the plural 'they' with 'anyone' in some cases? Does it substitute and replace 'he/she'?

note: this previous posts also says anyone is [singular]: "Anyone has" or "anyone have" seen them?

Answer

Singular They:

Merriam-Webster’s Dictionary of English Usage remarks that “They, their, them have been used continuously in singular reference for about six centuries, and have been disparaged in such use for about two centuries. Now the influence of social forces is making their use even more attractive.”