Can developers put a way on steam to have all versions of the game ? We can find on the net the 6.0 to 18.1 (demo)
Especially the 0.17 on steam when we bought one copy of KSP....
Do not say that's illegal because it will be added to all previously versions available on steam.
Unfortunately we can't get versions before 1.0.5. on steam.
I think you cannot get anything older than 1.0.5 on the Beta Program. But you can download them by brute force using the Steam Console.
You need to open a browser and then type on the URL "steam://nav/console" .
This will open steam with an extra tab, Console, on top right. From there you can type commands:
download_depot 220200 220201 7104947787544389563
This is the older manifest (the last big number) available on steamdb - if Steam has something yet older, it's not registered