Just discovered: You can actually "show website icons in tabs" right now, in
macOS 10.13 High Sierra. Probably not what this new "feature" refers to but it might be useful until you install
Mojave macOS 10.14.
You can reduce the size of a Safari tab by clicking in a tab and dragging left. (I'm sure a gesture can do this also) At some point, the tab will collapse to a tiny size, just big enough to display an icon for that site.
Now, I can have ~57 tabs even in my 1200 pixel wide browser window!!
