or somewhere I can download older versions of XD.
You can install an update that is one version older than the latest version using the Creative Cloud desktop application.
Is there a specific reason as to why you want to go back to an older version? You will not be able to use any files created in the latest version in an older version of XD.
I had XD version 12.0 & I erased it to upgrade. But I need to update my OS system to download the new version. But my laptop is not compatible for the OS System. Now I don't have any version of XD at all.
We are sorry to hear that you're facing trouble. I would request you to please check the system requirements to run Adobe XD in this article: https://helpx.adobe.com/in/xd/system-requirements.html
Let us know if that helps.
It seems that you did not read carefully the problem.
If the clients can update their OS, they can install the latest Adobe XD. But the problem is that their hardware is outdated and the new version of OS can not be install.
So your template answer does not help.
What a wonderful support!