Do you know that for certain? I always suspected they would, so they could transcode to better formats in the future, but never found anything to confirm it.

I know it was true at least a couple of years ago, but there is no guarantee that they keep it.

On all of the videos I have uploaded to my YouTube channel, I have a "Download original" option. That stretches back a decade.

Granted, none of them are uncompressed 4K terabyte sized files. I haven't got my originals to do a bit-for-bit comparison. But judging by the filesizes and metadata, they are all the originals.