Hello, I bought Celeste as part of the bundle, so I didn't receive a Steam Key. I really don't like itch.io's library layout, so could I possibly prove my purchase to receive a Steam key instead?

No, none of the Bundle for Racial Justice games come with steam keys (possibly so they're all DRM free)

You can however download the game and then add a link within steam to launch the game from the steam launcher.  https://support.steampowered.com/kb_article.php?ref=2219-YDJV-5557

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