Here's what Google says:
Custom tabs are now called Custom Widgets.
The requirements have changed since the last time custom tabs were implemented.
Please Note: The Custom tab is enabled by working with specialists on your Google Team known as Technical Producers. Please be advised that minimum investment levels and other requirements apply. Unless enabled by your Google Team, you will not see this section in your Channel Settings.
URL: Google YouTube Help pages https://support.google.com/youtube/answer/1727240?hl=en