Disabling One-time upgrade prompt when a critical vulnerability is detected upon login

Dear @heyyo , 

Do you mean hiding the pop-up notification ? because you're not forced to upgrade in this case , you can skip the upgrade as shown in the screenshot on this article.

BR.

https://community.fortinet.com/t5/FortiGate/Technical-Tip-Understanding-the-FortiOS-critical-vulnerability/ta-p/426944 

The only way (that I know of) is to disable the security rating check - 

config system global

set security-rating-run-on-schedule disable

end

And additionally to clear the existing detected vulnerabilities - 

diagnose report-runner vuln-clean

This is a trade-off but the only option that I know of.