Probably 17.08.1 (Android 6)
Do you use Fairphone OS (with Google Apps and services) or Fairphone Open OS (without Google Apps and services)?
Fairphone kept Fairphone OS 18.04.1 (Android 6) around … https://support.fairphone.com/hc/en-us/articles/213290023 … there’s a link to installation instructions, too.
Fairphone Open OS doesn’t have a page like that from Fairphone, only the last Android 7 and the latest Android 9 release are available here … https://code.fairphone.com/projects/fp-osos/user/fairphone-open-source-os-downloads.html
The forum has old Fairphone Open OS download links here, though … ✏ FP Open / Sibon download link archive
What do you mean with “old models”?
You could opt for new camera modules, and there were some small revisions over time needing OS support apparently, but the Fairphone 2 is the Fairphone 2.
Your concern got you a display with 0% performance. Sorry, couldn’t resist 
.