There is no law (that I'm aware of in the US) that requires Google to purge any user data except GDPR (which applies only to EU residents after May 25, 2018), or perhaps the EU's "right to be forgotten" (which isn't going to supersede preserving evidence for an investigation).

Those Youtube videos exist, if not with a "is_deleted" flag, in Youtube backups. Some are already in the Internet Archive.