No it wasn’t really discussed. It wouldn’t be very easy to implement. The excuse I have given myself not to do panes (because the complexity of doing this properly) is that you can use windows side by side too. Or a browser that supports splits natively, like Vivaldi for instance.
Yes, in PWA mode you can also create new windows. And on mobile (phones at least) you don’t really have physical space for panels. On iPads you do, and there you can use the OS’ native split views again.
Gravedigging a bit here… I agree that it’d be a challenge to implement for sure. That said, I agree with @pihentagy here in that it would add a lot of value. One main advantage of handling split views within SB instead of relying on the browser / OS is that the user would be able to customize hotkeys and such for tabs, as well as have the potential for plugins to do fancy things in a pane.
Ultimately, I would love the ability to stack tabs like you can in Obsidian, but I don’t envy whoever tries to implement that