This is what I received from Trimble
Hello Claire,
Thanks for reaching out to SketchUp Support!
We will always recommend using the latest version of SketchUp, and that’s available through a subscription. However, you can always continue to use your current version of SketchUp. The Web Services features affected by our End of Support policy use outdated embedded web browsers that cannot be updated and provide a lower quality of service and security. Those older features designated as Web Services present a potential security risk, and we made the decision to disable them to better provide a consistent, reliable, and secure experience. Please see our end of support policy https://help.sketchup.com/en/admin/end-of-support-policy
Even though the Maintenance & Support has run out on your SketchUp Pro Classic license, you can still always ask for support from the SketchUp Community Forum at: http://forums.sketchup.com.
You will need to update to a supported version of SketchUp to be able to install Diffusion.