Right, the graphic here was not necessarily for an exact UI, but more just a visual representation of the concept.

Most OSes, however, do in fact use a lock icon next to the WiFi icon to denote a secure network, albeit perhaps not necessarily overlaid. This includes at least macOS, Android, Ubuntu, elementary OS, and GNOME. Windows uses a shield icon instead, but is the same idea.

My suggestion is to move away from denoting “secure” networks as OSes already do, and instead to denote insecure networks. Regardless of the exact iconography.