they are just “collectathon stuff” so you can proceed the story without, all you get is “bonus stuff”, but your leaf suit is just fine without 
but like maoski said the game lets you walk past the pyramid without just fine to continue your mission
that said, once you are at the point of the pyramid, you coooouuuld just grab a guide to get the medallions quicker now (some are indeed hidden very well) since by then you should have all the necessary upgrades to get to all places
it’s not "metroidvania"metroidvania, but there are indeed backtracking in this game to get all collectibles, as a bunch of places are “gated off” by requiring Xupgrade to leaf suit to traverse,
but again all you get in the end is merely “bonus stuff -yay cheers you collected all stuffz” and some collecathon style achivements as i recall. The campaign itself you can blaze right through just fine (pretty quick even if rush it)
now, don’t ask me if the bonus upgrade is worth the backtracking, -because i honestly can’t remember what it did 
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