No GBA flash carts were ever blocked by anything that officially runs GBA games. The TV adapter made for the original GBA will not fit in a GBA SP and likely newer devices but there are no software blocks on anything.
Unofficial things with GBA slots may vary. Many are dumping hardware connected to emulators so while that works well for just about every official game ( https://mgba.io/2015/10/20/dumping-the-undumped/ for the exceptions) most flash carts will not work by virtue of loading one section, selecting a game and then loading another, and the dumper will therefore only dump the loader. The exceptions will be really old school NOR era carts if you load them without a loader and possibly the expansion packs from the DS slot 1 era.