It's still highly used in the EDA industry - almost every EDA tool is scriptable with Tcl. There are even IEEE formats that are actually Tcl dialects, e.g. UPF (Unified Power Format) for describing power related features of circuits.
I wonder if the distinction of SubLists and SimpleLists makes static code analysis and syntax highlighting more viable than in Tcl where you cant distinguish between "code" and "data"...