When Imagick is loaded, it checks the version number of ImageMagick that it was compiled against, with the version number that is currently being used and will give a warning if they don't match. This warning can be suppressed by enabling this ini setting.
Using a version of Imagick that was compiled against a different version of ImageMagick than the one being used is not recommended. Although it may appear to work, it can lead to random crashes or other undefined behaviour.