is there a supply chain attack possible? What if some time later this list gets updated?

I will update it later down the line as YT changes its layout and for bug fixes... but i think this was not what your question.

I don't think that something super bad can happen with these uBlock filter, they will sanitize the filter heavily.

Maybe a potential attack vector for these lists in general is to hide the body of a few sites but this is more annoying then dangerous AFAIK.

That's the benefit of Brave Browser. The YouTube Shorts blocking feature is built into the browser itself and is not from a third party.