iTunes Radio shows featured stations at the top of iTunes Radio. These are regularly updated and thus 'bumping' older featured stations. Some of these stations can still be accessed through the add station button. However some cannot be accessed except through an iTunes url that someone has shared.
For example here is the text of a tweet sent with a link to an old iTunes Radio featured station that I can't find through the add station button.
Tune in to Music Heard at WWDC on iTunes Radio: https://t.co/a5xbTAAI8m
Is there a list of links to old iTunes Radio featured stations?