For a long time I have not used the standard Reminder application and even deleted it from the iPhone. Sometimes from this program notifications come, what to do?
Back in iOS 11, developers allowed users to remove most of the standard programs from the iPhone and iPad. In the previous version of the operating system, applications were trivially hiding from the desktop and did not free up space on the device.
Please note that the Reminders application is one of the applications that allows you to store and synchronize data between devices via iCloud.
Most likely, before deleting the program, you left a few reminders in it with a trigger by date or geo-position. So reminders are downloaded to the cloud and issued on the smartphone even after the program is uninstalled.
To turn off notifications you need to install the Reminders application and delete unnecessary entries in it.
You can use the web version of the iCloud service and delete the reminders there.