Don’t worry it’s not just you (Desktop client's changelog problems · Issue #1123 · owncloud-archive/owncloud.org · GitHub) We are working to bring old versions back to the changelog (as they’re also important for troubleshooting as you mentioned)
Right now, the easiest way to get an older version is to build it from source: Releases · owncloud/client · GitHub. There you can get the particular version you want and compile it for your system. Might take a bit more, but you’ll end with the original version you need.