Stash should still be kept. It did its job and it did it well ( well, except for this: https://forums.opera.com/topic/4810/search-issues-in-stash/1 ). The 2 features compliment eachother.
Other browsers have addons for such a thing, to temp-save a link for reading later. Without it being lost in your bookmarks (even if you make an ugly "temp" folder). That's why people loved the old Opera. Tons of handy features built-in, no need a third-party addon.
There are several topics stating they want Stash back. And perhaps if they keep it, people might adapt to it and "move on". Something along the lines of what leocg said.
So why remove a feature when they have different purposes? Keep the innovative new feature and let people get used to it.