We already have Ctrl+Shift+N to reopen last closed window, so adding same functionality to Ctrl+Shift+T seems to kill its initial design.
Having multiple windows allows for separate workflows, and previously it was possible to use Ctrl+Shift+T to split them by using multiple windows, with each restoring only its contents. Also it was possible to remove a tab from that workflow by getting it out as a separate window and closing it there, thus not adding it to the closed tab, uh, queue. That helped a lot with opening - for one - messenger links that one weren't interested in keeping open or reopening.
The new mode seems to be popular with some users, though, so why not make it a toggle in settings?
Some points are also mentioned in the following discussion:
https://connect.mozilla.org/t5/discussions/command-shift-t-shouldn-t-open-last-closed-window-it-shou...
Some more points are mentioned in the following Idea's latest comments as well:
https://connect.mozilla.org/t5/ideas/shortcut-ctrl-cmd-shift-t-to-restore-previous-session/idi-p/221...