Welcome screens have been used before in the Ubuntu world, by the Ubuntu MATE and Ubuntu Budgie official flavors for example, and are also being used by numerous other GNU/Linux distributions out there to provide a one-stop solution for setting up your freshly installed operating system.
Ubuntu itself never used a welcome screen, but with the forthcoming Ubuntu 18.04 LTS release things change in this regard. The new welcome screen in Bionic Beaver will help new and returning users better understand how the brand-new GNOME user interface works, as well as to set up things like Canonical Livepatch.