It pops up in a bunch of different areas. If you used the Orbix Corba ORB, it was there. It's the scripting language for F5 load balancers. A one-time popular monitoring tool called NetCool/Omnibus used it for script extensions. It's also in a few different Electronics Design Automation (EDA) tools. It was also in a few different tools that dealth with Healthcare-specific IT, like HL7 protocol bridging. Just the ones I ran into. Prior to lua it was really the only dead-easy way to link a known, mainstream-ish, scripting language into an existing tool.

We're using it now (2022) to run some high volume transaction websites. We are not writing new apps in it, but a lot of existing code still uses it, and is actively modified.