Honestly, this is a terrible choice! I like Lua, but it's not mainstream, and it's not going to be! There are tons of embeddable JavaScript engines - they could've used TypeScript, too!
Lua support is NOT being added for templating. It's being added to enable extensions in charts (a new thing) and for cross platform plugins. Existing style plugins will still work. This is providing some extras to help enable more cross platform work.
There are a couple reasons Lua was chosen and documentation has been written up as well.
1. There are go packages for this so we can do it in a cross platform manner (mac/win/linux/bsd/etc). Helm is used on many platforms.
2. Security issues, especially with extensions to charts, have a path forward to address. In a similar way to how we opt-in for apps to use features on cell phones we can do that for features used in extensions.
The people who did the analysis has embedded JS engines in applications recently. They are aware of the benefits, pitfalls, and how it all works.