Currently the latest Notebook 7 pre-release only lists JupyterLab as an alternative interface for opening the current notebook:

When it was added in RetroLab it also displayed the classic notebook becausenotebook was installed automatically as a transitive dependency:jupyterlab/retrolab#309

Currently nothing installsnbclassic by default anymore so the "Open with Classic Notebook" entry is not displayed.

But maybe the interface switcher plugin should try to detect ifnbclassic is available, and add an extra menu entry that points to/nbclassic/notebooks if it is?