Is it possible to access versions Pre-1.0 (Alpha or Beta) of KSP though STEAM?

[Dr. Kerbal]

Is it possible to access pre beta version of KSP when bought through Steam? Because you can access version 1.2.1 and up but not 1.2 and below. SO it is possible. And how do you do it?

[swjr-swis]

You probably didn't scroll enough down the pick list; you can go back as far as 1.0.5. They leave only the last minor patch of each version, and they never added anything older than 1.0.5.

[Dr. Kerbal]

4 hours ago, swjr-swis said:

You probably didn't scroll enough down the pick list; you can go back as far as 1.0.5. They leave only the last minor patch of each version, and they never added anything older than 1.0.5.

Really. Brace for me they only go back as far as 1.2.1 now. I’ll go check though...

4 hours ago, swjr-swis said:

You probably didn't scroll enough down the pick list; you can go back as far as 1.0.5. They leave only the last minor patch of each version, and they never added anything older than 1.0.5.

Wow. I see 1.0.5 wow.

[Gargamel]

Steam access wasn't introduced to KSP until ~.21 ish.    Unless you already own a copy of a game version from between ~.13 and 1.0, it is illegal to obtain one.   On both the KSP store and Steam, Squad maintains the most stable release of (almost?) each major update from 1.x on, so you can get those.  

Read through this thread, and some of the threads it links, to find more info.

 

[4x4cheesecake]

Since this post got linked here, I want to mention that my method uses a different tool to allow the download of old game versions and I'm NOT going to cover the tool, mentioned in the linked thread.

It is possible to go back to the version of KSP, which were released since somewhere around september 2013, so probably 0.21.1.

As others mentioned, for some older version you can use the "beta" options but if you need a very specific patch version, you can use the steam console to get them.
Unfortunately, this became a bit more iffy recently, since valve more or less blocked this option in the console as well...anyway, since this a feature used by quite some people, someone found a solution to work around this. If they really don't want us to download games through the console, they would remove the whole command...wouldn't they?

READ THE WHOLE THING BEFORE ACTUALLY DOING ANYTHING!

First of all, you need to know when the version you're looking for got released. You can use the Version History for this. 
Then, you search for the entry of KSP on SteamDB and select the entry you need (KSP basegame or one of the DLC). Click on the corresponding AppID to get forwarded to the entry of the game.
You also want to note the AppID for later!
Now you have to select the depot you need (in this case choose the correct platform, like PC, Linux or OSX) and again, click on the ID and note it for later!
The last ID you need is the manifest ID, so click on "manifests" and you'll see every single manifest ID with a corresponding date. Well, they recently changed it that you have to be logged in on the website through your steam account to see the whole list but that shouldn't be a big deal.
For version above 1.5.1, you need to switch to the "win64" depot.

Well, here comes the iffy part: you need to download the SteamDepotDownpatcher from github: https://github.com/fifty-six/zig.SteamManifestPatcher/releases

I found this tool through the speedrun community, which often requires specific patch versions of a game to perform certain runs in a game. This tool will temporarily release the block established by valve and allows you to download games through the steam console again. It's a fairly simple procedure by changing a value in your RAM of the running steam instance. 
Windows will warn you to execute this program since it is not singed and therefore not trustworthy for windows. I'm not going to tell you to run this tool as well, you have to decide on your own if it is worth the risk. Since the source code for the tool is hosted on github as well, you can take a look at, if you want.

If you decide not to run it, I've wasted you time and there is nothing else I can do for you, sorry.

If you decide to run it anyway, we can continue:

First of all, you have to launch steam if it isn't running already. Then run the SteamManifestPatcher. It will open a CMD promot which shows you the changed value and if the operation was successful.
Now we have to open the steam console: open a run prompt in windows (press win + R) and type in "steam://open/console" (without the ""), press enter and steam will open it's console.
In the input field of the console, type:

download_depot AppID DepotID MenifestID

(of course you have to put in the IDs you've noted before)

For example: If you want to download version 0.90.0 for windows, this command would be:

download_depot 220200 220201 1535792782041218707

If anything was done correctly, you'll see the a red text in your console which is basically just what you've typed in, and below

downloading depot 220201 (xxx MB)...

You'll not see any progress bar for the download and steam will not notify you when it's done but you can see the network activity in the download section of steam. This kind of download is usually very slow, so don't expect to load at max speed, more like 300kb/s. Sometimes less, sometimes more.

Once the download is done, the steam console shows something like that:

Depot download complete : "C:\Program Files (x86)\Steam\steamapps\content\app_220200\depot_220201" (2155 files, manifest 1535792782041218707) 

You'll see the path to the files here as well, it's always <your steam install directory>\steamapps\content\app_AppID\depot_ID"

And you're done

The next time you close steam, the changes done by the SteamManifestPatcher are undone and you have to run the tool again if you want to download a game through the console again.

BTW, if that isn't clear in the first place: this will only work if you actually own the game. You cannot download games or DLCs through the console if they are not in your steam library. I have no idea if this works on a shared family library.
Also, you'll do this on your own risk I'm saying that because I can imagine that valve disabled this feature for some reason we don't know. If they ever decide to take actions against people/accounts who enabled it again, I'm not responsible for any kind of damage/penalty caused by this. I'm just providing the information how it can be done, everything else is up to you

Edited March 24, 2021 by 4x4cheesecake