I also hate this and don't know why we don't have the option to turn them off. I got fed up with it and found a way to get rid of all notifications. This means all group notifications, friend status, and probably achievements. If you want to use any of those, do not do this. You can reverse it easily though by just deleting the file you create.
What you have to do is create a skin for steam and disable the notifications through the skin. This sounds difficult, but it is very easy.
Open the steam folder on your computer and then open the skins folder. Now create a new folder here and name it whatever you want the skin to be called. I used the name (Remove Notifications). Then inside of that folder create another folder named "resource". Then inside the resource folder create a folder named "styles". On windows XP the location would look like this:
C:\Program Files\Steam\skins\Remove Notifications\resource\styles
Now leave that folder open and go to this folder:
C:\Program Files\Steam\resource\styles
There is a file in this folder named "steam.styles" (you may just see a file named steam, that is it). This is the skin for steam, but if you edit it here it will be replaced with the original the next time you start steam. Copy this file and put it into the empty folder I just had you make above.
Now you can edit this file in the new folder you just made to get rid of the notifications. Open the file in a text editor (notepad). Scroll down until you see these settings (or press "control F" to search) and change them to what I have shown here:
Notifications.SlideDirection "none"
Notifications.FadeInTime "0"
Notifications.FadeOutTime "0"
Notifications.DisplayTime "0"
Fade in and out time must be set to 0 because the display time does not take effect until the notification has faded in.
Save the file. Now open steam, click settings, go to the tab called "interface" and in the drop down box on the right select the skin you just created. Restart steam. Done.
You can look through that file and change other things if you feel like it. If you mess anything up, just delete the file you created and it will go back to the default.
What you have to do is create a skin for steam and disable the notifications through the skin. This sounds difficult, but it is very easy.
Open the steam folder on your computer and then open the skins folder. Now create a new folder here and name it whatever you want the skin to be called. I used the name (Remove Notifications). Then inside of that folder create another folder named "resource". Then inside the resource folder create a folder named "styles". On windows XP the location would look like this:
C:\Program Files\Steam\skins\Remove Notifications\resource\styles
Now leave that folder open and go to this folder:
C:\Program Files\Steam\resource\styles
There is a file in this folder named "steam.styles" (you may just see a file named steam, that is it). This is the skin for steam, but if you edit it here it will be replaced with the original the next time you start steam. Copy this file and put it into the empty folder I just had you make above.
Now you can edit this file in the new folder you just made to get rid of the notifications. Open the file in a text editor (notepad). Scroll down until you see these settings (or press "control F" to search) and change them to what I have shown here:
Notifications.SlideDirection "none"
Notifications.FadeInTime "0"
Notifications.FadeOutTime "0"
Notifications.DisplayTime "0"
Fade in and out time must be set to 0 because the display time does not take effect until the notification has faded in.
Save the file. Now open steam, click settings, go to the tab called "interface" and in the drop down box on the right select the skin you just created. Restart steam. Done.
You can look through that file and change other things if you feel like it. If you mess anything up, just delete the file you created and it will go back to the default.
Credit to : AllTracTurbo
