This is included in JupyterLab 4.0 under Ctrl + Shift + H (https://github.com/jupyterlab/jupyterlab/pull/14053).
As for why it is not H see this @fcollonval’s comment on PR discussion:
I fear using
His to simple to be robust in JupyterLab. And as the shortcuts help can be displayed on any document, it will for example be impossible to use it with the file editor. So we would end up having multiple shortcuts depending on the active document.But at least the infrastructure does not prevent us to use
Hin Notebook v7.
Similarly, for many shortcuts there will be a solid rationale for any departures from Notebook ones, but there may be some shortcuts which were just omitted and can be implemented if someone can contribute 