Opera used to have this feature, and it was great. It was way better than the current Workspaces, because you could quickly and easily make tab group "folders" on-the-fly. They never said why it went away, but if it wasn't because it infringed on a patent or something, they should definitely bring it back. Circa 2014 https://www.youtube.com/watch?v=VWpJvg8icmM
This was a much better worflow, because I usually have lots of tabs open for [many] various different projects. For example, I may need to repair something on my car and open several videos, parts sites, and how-to forums. Then I have a similar set of several tabs that I use at work, about a trip I'm planning, looking for a Christmas present for someone, sites I frequent for a particular hobby like skateboarding, etc. Dozens of sets of several tabs each. Workspaces doesn't really work well for dozens of these little dynamic groups. Tab stacking, however, was PERFECT! Bring it back please!