If when editing or creating new diagrams you see the alert "unsupported version, please upgrade", this means you are using a free version of draw.io for Confluence Cloud.
draw.io for Confluence Cloud went pay-for in August 2017 and we froze the code version at that time. That code is the integration layer to Confluence Cloud, the editor itself uses the latest on the draw.io web site.
At the point we froze the integration code there were no known major issues. However, the Confluence API is evolving, whereas, our calls to that API are not. It's possible that an Atlassian API is deprecated and removed and we're still calling it. Or the method behaviour might change.
In March 2018 we hit such a compatibility problem and introduced the warning to alert users that an issue had occurred. The issue related to versioning, we no longer display the correct versioned diagram in reverted Confluence pages.
Ultimately, the free version will break at some point and we will not fix it. You are advised to plan in advance to migrate to the upgraded version, or away from the plugin entirely.