using_pip/.jupyter/lab/user-settings/@jupyterlab/extensionmanager-extension/plugin.jupyterlab-settings

{
    // Extension Manager
    // @jupyterlab/extensionmanager-extension:plugin
    // Extension manager settings.
    // *********************************************

    // Enabled Status
    // Enables extension manager.
    // WARNING: installing untrusted extensions may be unsafe.
    "enabled": false
}