After my maintenance period expires, can I keep using an app?

This depends on your deployment.

If you have a Server installation, your license doesn't expire. However, support and updates won't be available until you renew your maintenance period. To do so, please log in to your MAC account at https://my.atlassian.com/products. If you're unable to renew it via the Atlassian Marketplace, please email our support team via: help@k15t.com 

Apps for Cloud and Data Center are licensed on a subscription, recurring monthly or annually. Support and version updates are provided as long as your subscription is active. Once your subscription is cancelled, you will no longer be able to use an app until you reactivate it at https://my.atlassian.com/products.