If your draw.io for Confluence Server evaluation license has expired it is possible to generate 2 more evaluation licenses. Go to https://my.atlassian.com/products, logging into the account that has your current evaluation if necessary, and find the entry for that evaluation.

Click it and it will expand. Below the license key there is text to the effect of "New Evaluation License". Click that link and follow the instructions. You'll end up with a license key. You need to copy that license key and go to the manage add-ons (BASE_UIL/plugins/servlet/upm), select the draw.io plugin, press the edit pen next to the license key, replace the license key with the new text and press "update".