If you use Developers Tool > Elements and look at the outline pane (instruction on how to do this: Obsidian CSS Inspector Workflow), you’ll notice that the outline pane is in a <div class=“workspace-leaf-content” data-type=“outline”>, so it should be possible to target it.
However, further inspection reveals that the header level information is not included, only the text of the header. This means that you don’t know which header level the text in the outline is at, and therefore you can’t hide only <h6> from the outline.