I grew up in Mississippi, and I had never heard of this! I can say that practically it doesn’t make much of a difference: once eighteen, you can still enter into binding contracts, execute estates, sue people, and buy cigarettes[0]. It seems to still mostly apply to drinking, but you’re right. Technically you’re a minor until you’re 21!

[0]: https://minors.uslegal.com/age-of-majority/mississippi-age-o...