It was designed to be embedded, has a very small footprint, is relatively fast ( we use luajit ), and has few opinions as a language. That said, there is nothing locking us in to lua permanently as we would just need to implement a small-ish set of language bindings and docker configs to execute the language du jour.
Shameless plug here, but if you're interested in voice platforms and are growing weary of Twilio, I'm working on a new hosted telephony platform that uses embedded lua executing directly on our hosted servers. Check it out - https://developers.corvisa.com/
PL is an enormous library so its really superseded by a few different libraries. The stuff I used most often were the functional tools and Moses (https://github.com/Yonaba/Moses/) blows the doors off of PL in that regard.
Can we PLEASE not refer people to penlight anymore? It's half broken, poorly documented, and poorly maintained. It drives me crazy to see it referred to all over the place as a good example of anything.
That being said, this is FANTASTIC. Due to the nature of lua it may well always lack a good community but thankfully people seem to be trying.
It has worked remarkably well and allowed us to scale up/down during peak hours or unexpected high traffic peaks.