In early June, YouTube migrated all channels to a new design called the “One Channel.”
Channel owners had a choice to switch earlier than June, but many people just waited out the inevitable because of some grumblings about the Youtube One Channel design. It used to be that channel owners could brand the entire background of their YouTube real estate, allowing for additional messaging and creativity. The brand image could include content along the left and right sides, top and bottom.
With the Youtube One Channel design, there is simply a header image. Everything else appears to be rather similar to every other branded channel. However, with this new design your brand image is now also visible on iOS & Android apps, the mobile web and via the television (for people who watch YouTube on the boob-tube).