You can create a css file like the following:
div[data-path$='assets'],
div[data-path$='assets'] + div.nav-folder-children
{
display: none;
}
div[data-path$='.png']{
display: none;
}
Replace assets and .png with the folder name and extension that you want to exclude.
This is a workaround until such a feature may be integrated in some years.
(Usually you would setup this once. So that’s a workable workaround)