The LuaRocks website is pretty small.

Lapis was born from https://itch.io (kind of like Rails was born from Basecamp). It is still the "largest" website I know using it.

OpenResty itself has a similar story with Taobao, one of the largest wesites in the world, even though now they use a fork (tengine). I think several other large Chinese and Russian websites use it.

In the US, Kong is also based on OpenResty, and CloudFlare uses OpenResty a lot (and is now its main sponsor).