- Download and install Debloater.
- Check "3c_ota.apk" (com.motorola.ccc.ota, "base.apk" on some versions) and click apply.
Right after done this, the annoying and dangerous notification "Install System Update" is gone, forever.
In order to block some other annoying notifications: "SIM card is not from Verizon", "Sign into network" and "Update package notifications" (by WeChat), you may simply install an app called as Notification Auto Clear. Note that this app will block all notifications by default and you need to enable those that you do want.
Other than using a simple app, I do it in a far more complicated way: Tasker + Notification Listener.
In Tasker, I create a new profile: Event, Plugin, Notification Listener, Posted and select all apps.
The new task for this profile: Plugin, Cancel notification, and "Notification key" is %nlkey. Then add all the titles of the notifications you want to block in the "If" field.
I also disabled all the apps I don't need: Settings, Apps, ALL. Disable all you may, except those you do need. Don't disable these three unless you know what you're doing: Google Play Service and Google Service Framework (for Google apps such as GMail and Google Voice), and Media Storage (for ringtones). You may use Debloater, but it is more powerful and thus might let you disable wrongly a necessary system app.
See how clean the notification area is now:
Lastly, if you're bothered by the phone's self activation, check "VzwPhoneService.apk" in Debloater and apply.
Enable GSM mode on Verizon Moto E 5.1