Ash here from the Shopify team.
Thank you for reaching out with your question! I'm happy to help out. When the free trial ends, if no plan is selected then the account will go into the frozen state. A frozen account is essentially closed, but can be re-activated upon selecting a plan and adding billing information to the account. If you no longer wish to use the account, then you can simply leave it in the frozen state, and it will eventually be marked as a dead shop and the information will be removed from the platform.
Let me know if you have any further questions about this, and I'll be happy to help out.