Title

does it really cost you anything to say “original project by @guy” though

Maybe they feel extra grateful for you or they credit as backup in case the original project gets taken down (when the original project gets removed, the remix banner is also removed with it)

because it's just basic kindness tbh
yeah you don't HAVE to but it's really nice if you yourself acknowledge it rather than just the automatic banner

bruh I experienced this before
when the original creator said to give credit
then I replied with “remixing automatically gives credit”

THEN ST TOOK DOWN THE PROJECT ANYWAYS
—
anyways
basically, if the original project is gone, no more auto credit

I think it's just good etiquette to mention the original project creator. It shows that you have acknowledged the effort they put into the original project.

It's nice to give credit in the Notes and Credits section as well, but you don't have to – according to the Scratch Team, the automatic credit is enough:



(source)