So unless you're already on the old layout, It looks like there is [no way now to revert to "Classic. To exit the Material Design layout, you can go to YouTube's homepage, click your channel icon in the top right to pop-up a menu, then click. Chrome * Open a Youtube page * Press Control+Shift+I (that is an I as in information) * Application tab on top (may have to click the arrows to view more tabs).
Remember the old days of YouTube? It looked so different. See if any of these old features conjure up any pleasant memories of simpler times.