For a while, I know that https://flightaware.com/ used it fairly heavily. Indeed, their careers page still shows an interest in Tcl skills.
After he sold the company to Red Hat I worked there really briefly. Apparently half the sales fell through because Tcl wasn't regarded as "enterprisey" enough for customers (note the customers themselves would probably never write a line of code). There was an attempt to rewrite the entire engine in Java — it being 2002 — which limped along for a while before the project died.
But there's more! The original Tcl stack was open sourced and continues to this day as OpenACS (https://openacs.org/).
The Java rewrite also still exists (https://svnlegacy.ow2.org/byline/) but has been dead 15+ years, and just digging into those 7 level deep directories rekindles a feeling of dread in me.
ad_proc and online documentation was awesome.
I also had to figure many things out from first principles because documentation was all for PHP and Apache, not aolserver and tcl.