Weinstein's predatory behaviour is an open secret is believed to have been known by a majority of Hollywood A-listers. It's likely that Meryl Streep, Tom Hanks, George Clooney, Ben Affleck etc. all knew but ignored it.
Keanu is one of few Hollywood stars who's universally admired for his kindness, he never hung out with the big stars and other than Laurence Fishburne, most of his friends are ordinary people. I think it's highly unlikely he knew, he's never been a part of that crowd.