Hey Giovanni,
The sidebar isn’t a plugin, it’s part of the core of Discourse.
These are all configurable via /admin/site_settings - search for sidebar to see them
If you’re referring to the location to customize the contents of the subscriptions banner, this is done via your Customize → Text at /admin/customize/site_texts by modifying js.discourse_subscriptions.campaign.title and js.discourse_subscriptions.campaign.body.