I had played Agony Unrated in late 2021 and hadnt touched it since. Then I wanted to install a different game and uninstalled it, only to find out that it completely vanished from my library.

According to news posts from the the developer, Madmind Studio, sometime mid February of 2022 Agony Unrated disappeared from many players libraries.
Steam de-linked Agony and Agony Unrated and you can find more on that here and here.

The good news is that the developer found a solution to this by adding Agony Unrated as free DLC.
A simple way for pretty much everyone to regain what they lost.
If only the Agony Unrated DLC wasnt region blocked for Germany.
You see, this means that German customers cant access the Store Page for the DLC and as such cant add it back.

Naturally I messaged Steam Support about this and to the surprise of no one they couldnt help me. I honestly think they didnt even understand the problem.
They suggested to get in contact with the developer of the game.

Naturally I e-mailed them, purchase receipt and all. Weeks passed and I was only met with silence.
It took several months for them to reply to me but they did provide me with a code to add the DLC back but at that point I had already solved the issue by myself.

Trying to contact Madmind Studio directly seems to be the only option left right now.
I used the contact form on their website. http://madmind-studio.com/

Since the official channels for help were not helpful at all, it got me thinking.
How can I solve this problem by myself?
Then it hit me. I already used the solution for this before on a different game.
So its time to take it into our own hands! Note that this method only works with free DLC.

Open your preferred internet browser and navigate to your URL bar.
In this example I used Firefox Nightly.

Now copy "steam://run/1921050" so it looks like this.
To get the ID I went to SteamDB,[steamdb.info] looked up Agony and navigated to the DLC.

Once you press enter Steam will try to install Agony if its not installed or most likely start downloading files necessary to start up Agony if it is installed already.

A fresh install with the DLC enabled should come down to 24GB in total.
Upon trying to launch Agony you should be getting an extra window in which you can choose what version you want to play.

In case Steam does not display this extra selection window you should
also be able to directly start Agony Unrated by going to the install directory
\Steam\steamapps\common\Agony\UNRATED
and use the Agony.exe found in that folder.

This means that it worked and you successfully added Agony Unrated back to your library as DLC.
Once you open the properties of Agony in your library you should find Agony Unrated under the DLC tab and it will most likely display the original date you got it at.

By going to your Account Licenses however you can find the actual date it was added back.

Congratulations! You have reclaimed what was rightfully yours to begin with. You beat the system!