SUMMARY
If multiple Dolphin windows are open, and then one of those windows is closed, there isn't a way to automatically "undo" the action of closing that window. That's no real trouble if the closed window had only one tab, but if that window had several tabs open and split views in use, then it becomes a bit more involved to find your place again.

The wish here is for a feature that saves the tab and split view contents of a Dolphin window when it's closed, and adds that window to a list in a Go > Recently Closed Windows menu. The listed items in that menu would each contain the labels of the tabs that were in a previously-closed window, truncated if it was a larger number - for example, "Documents" if it was just one, and perhaps something like "Screenshots, .local, funny-memes & 7 more..." if it was more.

There was a previous bug filed that's somewhat related - https://bugs.kde.org/show_bug.cgi?id=472458 was closed because the feature was working as designed, although I don't necessarily know that the features are in conflict as described there when multiple windows are open, and the action that would have needed confirmation was not closing the last remaining window.

Regardless of the outcome of that one, however, I think creating the ability to undo an action would be a nice improvement over relying on a confirmation dialog.