I don't know how reliable the site is, I saw an article on GT Planet about it.
https://segmentnext.com/2019/03/27/rumor-ps5-to-introduce-ps-plus-premium/
According to an anonymous EU based game developer who is working on a PS5 launch title, PS5 PS Plus Premium will offer players guaranteed video game betas and alpha access as well as the ability to create private servers.
Apart from these two features, the developer didn't mention anything else. However, he did confirm that PS Plus Premium will run parallel with the base PS Plus. Sony isn't attending E3 2019 so we expect PS Plus Premium details during a State of Play stream or at PlayStation Experience 2019.
Meanwhile, the reveal of PlayStation 5 will be in Q2, 2019 claims the developer. The PS5 reveal will be minor, probably a tease, he says. PlayStation 5 release date is set in March or November 2020.
https://segmentnext.com/2019/03/27/rumor-ps5-to-introduce-ps-plus-premium/