Apps for Confluence Server have perpetual licenses and draw.io is no different. That means you can download and run any version of draw.io released during your maintenance period. Versions released after that period you cannot install and run.
Assuming you take the last version released during your maintenance period, you are licensed to run that forever. But note that we only tested it on the latest Confluence version available at the time of draw.io release.
After your maintenance period expires, you are no longer entitled to support for draw.io.