There's a way to still do the rm -rf / bypassing the warning but you shouldn't do that.
Ever since systemd was a thing, that command has stopped being 'safe'. It no longers solely affect the filesystem. It can wipe your EFI variables and make your comnputer unable to boot at all, even unable to boot installers to reinstall linux.
https://github.com/systemd/systemd/issues/2402
Don't think of the file system as just the file system. If you keep thinking of / as only meaning 'whatever's in that hard drive' you will not like what you may encounter.