Although metroidvanias are typically platformers, genre isn’t what defines a metroidvania so much as the non-linear progression and upgrades. Also shooter platformers do exist like, say, Metroid.
i think there are plenty of ways that shooter elements could be incorporated into the metroidvania style. to start with in metroid your main mode of combat is shooting. and there are a few i've played that even have boss fights designed with shmup style patterns.
Metroid Prime was basically fps and is probably my favorite Metroidvania I've played. You can make almost anything Metroidvania, it's all about having the progression directly tied to upgrades made to the player. Like upgrading the player's normal blaster to a flame blaster that can be used to melt an ice obstacle, unlocking a new area to the game.
