Seeing as the original YouTube Private Messaging system is no longer a feature (and had become obsolete), adding a business email to your "About" page is a splendid idea. Prospective sponsors and even other creators can easily reach you.
However, what I've seen some creators do is for security purposes, not list their actual GMail address associated with their channel, but create a new email address and simply add "
.busniess@domain.com" to the end of it (e.g - "
creatorxyz.busniess@gmail.com"). This is in order to weed out the spam and not publicize the email linked to the YouTube account. Just an interesting method I've seen some channels use.
Also,
@sarahja, while it again has become obsolete, YouTube has a new messaging system called "YouTube Friends" and it works similarly to Skype in a sense. You must first invite people before you are able to message them. Once logged into YouTube, click the "Messages" icon
and then the click the blue "FRIENDS" text, then click the blue "ADD FRIENDS" button and you'll be provided with an "INVITE LINK" to share with whoever you wish to chat with on YouTube. You can read more about it here. --> support.google.com/youtube/answer/6401182
In my opinion, sending an email to the creator is far simpler. Cheers.