You will soon be able to submit your t-shirts to the Shirt List again, but the submission policy is being changed. Details are coming soon. To be notified of when they are open, you can sign up for the Shirt List Weekly Newsletter.
The Shirt List has become quite large over the years. There are currently 36535 t-shirts on the Shirt List and counting with new tees being added every day. This has meant upgrades to servers and higher costs. Because of that preference will be given to stores with affiliate programs. And of those stores, preference will be given to stores with non-native affiliate programs such as Shareasale and CJ.com.
Link: Shareasale
From now on the Shirt List will promote only graphic t-shirts with an emphasis on pop culture.