I knew about it, it wasn't that rare of a practice in the GBC/GBA overlap years. Shantae had an entire transformation locked behind 'Advanced' Magic.

I remember this bonus being pretty heavily advertised in Nintendo Power when the games came out, it was obviously a marketing gimmick to get you to upgrade

I didn’t know that it was GBa exclusive until years later. I just through the shop just closes at times

The GBA shop. I did know about it!

I remember having trouble getting in there at some point, but I was aware of it either due to the strategy guide or GameFAQs. I think on more recent plays, I've checked it out.