It is possible to download versions as far back as 0.19, though the process seems somewhat complicated.
In the thread there is a comment by Lisias, which will tell you how to download much older versions.
I'll try to simplify it here:
1. Open browser, and type steam://nav/console
2. Steam will open as normal, but with a new tab on the top right called "Console". Click it
3. In the console, type "download_depot 220200 220201" but do not press enter yet!
4. Go to https://steamdb.info/depot/220201/manifests/ in your browser
5. Scroll down to the previously seen manifests section. You will have to log in with your Steam account to see old manifests.
6. Look for a manifest dated 7th of October 2014 (0.25 release date)
7. To the right of that is a long number called the manifest ID, copy that
8. Go back to the console, press space and paste the manifest ID.
9. Press enter. It will begin downloading. Once the download is finished, it will tell you where the files were downloaded. Go there
10. The folder should have a KSP.exe inside. Open that and you should have an ancient version of KSP ready to play.
Hopefully this was helpful. If not.... sorry
