I'm unable to sign in, it says "PlatformNotSupported" after the accounts update.
The itch.io version of Among Us isn't being supported. If you bought the game here, you have a Steam key on the download page. If you're not ok with using Steam (some can't, some won't, for various reasons), contact itch.io for a refund (the devs aren't the seller, so even though it's their decision that's invalidating your purchase, it's the store that's responsible for handling a refund).
If you want to reach out to the devs as well, more people doing so might help push them to actually support the version of the game many of us paid for.