How to access past DrupalCon website that requires password?
The reason they're password-protected is due to a critical security vulnerability in the version of Webform run on those sites. There's an open issue about it, and only the DrupalCon sites updated to a secure version of Webform are allowed to remain up.

In order to gain access to the ones that are still in lockdown, I'd comment on that issue to see what you can do to help get them updated. Otherwise, you can use the Internet Wayback Machine to access Archive.org's cached copy.