alt Hacker NewsGP isn't saying you should never have seedable random generators, just that they should not be part of the standard library because then the API promise is no longer that you get random numbers but that you get a very specific sequence of numbers which fixes the implementation as part of the API contract.
They can certainly be part of the standard library, it's just that you have to make the programmer be specific: it's a bad idea `new Random(seed)`, because you can then never update the `Random` class, and you might get stuck with a terrible default forever. But having a `new Xoshiro256(seed)` is a fine thing to have in your standard lib, given that quality PRNGs are such a common need.