Crucially, this was a project—a ROM created by a fan, not Mojang. Only a few hundred people ever downloaded the pre-alpha source code. This is the closest anyone has come to "Minecraft on GBC," but it is incomplete, buggy, and requires a cartridge flasher (like the Joey Jr. or GBxCart RW) to play on real hardware.
Copyright law is clear: Downloading a ROM of a copyrighted game you do not own is illegal. Since Minecraft never existed on the GBC, there is no copyright holder for that specific ROM. However, distributing Mojang’s code (textures, name, logo) without permission violates intellectual property law. If you download a fan demake, the legal risk is low, but if you download a file that uses Mojang’s official assets, you are infringing on their copyright.
If your goal is to play Minecraft on a handheld that feels retro, you have legitimate options far superior to chasing a phantom ROM.