Extension has no effect when using Notebook v7 interface instead of JupyterLab interface

[sjdemartini]

Most JupyterLab v3/v4 extensions have support for both the JupyterLab notebook interface as well as the Notebook v7 interface. (e.g. the NB v7 extensions page mentions this and also links to jupyterlab-skip-traceback in the page describing the transitioning from Classic Notebook to Notebook 7.)

However, it seems this extension does not support Notebook v7. For instance, here's the same notebook opened in the Lab interface (shows the collapse button) and then in the Notebook v7 interface (no collapse button):

Is it possible to extend this so that it works for Notebook v7 as well? Thank you for building this functionality!