MBC30 is a variant of the common MBC3 mapper that can access twice the memory as the regular variant. MBC30 allows for up to 4mb ROM and 64kb SRAM over MBC3's 2mb ROM and 32kb SRAM. It is only known to be used in the Japanese version of Pokemon Crystal, mainly for Mobile System GB features.
The added space provided by this variant is considered beneficial especially for ROM hacks. For example, Pokemon Coral (2022 demo and up) uses it for sampled cries and other extra content.
Everdrives will not work, unless an OS update is released. Until then, you'll need to make/buy an MBC30 flashcart (such as this project, or BennVenn's custom cart), and have reflashing tools.