The old Welcome page would open automatically when VS Code started, and allowed you to configure your SDK and toolchain version, as well as find resources like documentation and nRF Connect for Desktop.
We found this to be a little intrusive, and with the configuration options at the top, this page ended up being more of a configuration window than a welcome screen.
The functionality you'd previously find in the Welcome screen has now been moved into the sidebar's Welcome section, where it's a little less intrusive and a little closer to the rest of the extension's functionality.