Here's the changelog which details that slime block levels were decreased: Skyblock Update - January 1, 2020 | Skyblock Forums Gold was one of the first things to be decreased, but from what I understand with its relative comparison to other blocks which have also been decreased, it now is valuable again.
Gold Blocks have been nerfed quite abit, but are still more worth using for upgrading island levels as compared to slime blocks. If I'm not wrong there is also a Zombie Pigman mob spawner for free usage on Haberson's island. Hoped that information helped you
