MacOS 10.13.5 - Updated yesterday, getting spammed with System Extension Blocked
I had the same problem. This answer on a different copy of this question is a solution with a tradeoff: https://apple.stackexchange.com/a/327816/292890

Summary is: cmd-R on boot to get Recovery Mode, then open Terminal by clicking on Utilities > Terminal. Once open, enter the following command:

spctl kext-consent disable


It disables kernel extension security confirmations altogether, which of course hurts your security, but it does prevent all the confirmation popups. That's also less drastic I think than disabling SIP entirely.