Alebo trosku viac advanced postup

I've found a better answer!

Go into steam's folder in program files and find the "steamui" folder. In it should be another file called "css" that contains a "libraryroot.css" file.

Somewhere in that file is a line of code like this:

.libraryhome_WhatsNewContainer_gdZT7 {
position: relative; }

To hide the "what's new" banner, edit those lines like this:

.libraryhome_WhatsNewContainer_gdZT7 {
height:0%;
opacity:0;}

The lack of spaces in the modified version is important! Steam checks to see how many bytes long the file is to verify it, so the edit needs to result in a file that is exactly the same length as the original. Not adding extra spaces frees up enough space for the longer test and tricks steam into thinking the file is valid.

Unfortunately the file will probably need to be changed every time an update to the library UI is released, but at least it works.