Why people do like to copy Vivaldi solutions here I do not understand. Use the damn Vivaldi if you so much cherish and like, not paying real attention to the Workspaces solution. Stacking in Vivaldi is the least productive option I've met for many years.
Workspaces are far more visionary and efficient than the ungainly Vivaldi's Stack. But one has to have the deep need to use Opera with all it offers (as it is now), not turning to another wishful thinking list and trying to change Opera into Vivaldi Bis.
Vivaldi are far from the first to have tab stacking/grouping though, hell Opera had it themselves back in version 11.0. It's really not about copying their features, it's about implementing useful features to enhance the browser (and something Opera once had). Opera's old implementation was also quite a bit different from Vivaldi's using a collapsible design to show only the active tab with a small arrow button next to it to expand the rest of the group, have a look at this image below I just took using Opera 12.18 for what it looked like with tabs in both horizontal and vertical positions (something Opera is still sorely missing and a killer feature for me) -
Also if you enable popup thumbnails when hovering over a tab you could quickly & easily view all the tabs in the group and switch between them by clicking their thumbnail without having to expand the stack. Vivaldi does this as well which is its implementations only saving grace that actually makes it usable, they just need to improve the design of the tab groups in the tab bar which is just terrible and pales to old Opera's collapsible design.
And while workspaces is indeed a very useful feature you cannot argue that tab stacking like this wouldn't also be very useful in its own right, both can easily co-exist and work together to complement the user experience in different ways. The existence of workspaces is no reason not to implement such tab stacking.