I have Renshuu both in an iPhone and an iPad, in their respective latest OS version, 18.5, and I have never received system notifications.
Until now I thought that that was because it only used the internal notification system, with the envelop in the menu, but I recently saw that that shouldn't be the case. I have notifications enabled at system level and app level in both devices, I tried deactivating and activating both, forcing down the app before each change, but still no difference.
At this moment in time I have the test version of Renshuu installed on the iPhone, and the regular one in the iPad.
It's not a priority for me to get notifications (I don't really mind them much), but I thought it could be relevant for other users if I'm not the only one experiencing this problem.