There was a bump out, but I don’t think it physically stopped you from putting the cartridge into the console, I don’t have a GameBoy on me to check right now though. You’re right about the missing notch though, but that would only stop you from starting it on the original Gameboy and not the Gameboy Pocket or Super Gameboy. I know for sure I saw the Pokemon Crystal error screen when I tried to play it on the wrong device after being able to play Pokemon Gold on the older hardware.
Maybe I’m the one misremembering?
There were a few games that worked on the OG gameboy, but were GameBoy color games, the ones that come to mind first are Pokemon Gold and Silver. You could play them on the og gameboy, without color and missing some hardware features, or you could play them on the Gameboy Color as they had intended you do. It was a really nice addition.
I always thought of the GBC as its own generation, since the games were mostly incompatible, just as I thought of the GBA as its own generation for the same reason, but really, it’s almost just arbitrary.