It’s not “Instead of” though: .panel-body is a class targeting all panel bodies, while .hamburger-panel is targeting, well, the hamburger panel.
So I don’t know what specifically changed in core that broke your previous code. But if you want to hide all subcategories from the hamburger menu this would seem most straightforward to me:
.hamburger-panel .category-link.subcategory {
display: none;
}