far fa-pencil-ruler is a paid icon (the solid version is free). Free parts of the regular pack work just fine - see https://fontawesome.com/v5/search?m=free&s=regular for a list of them. It's much more limited than the solid free pack though (151 icons vs 1001), so that's probably why you ran into this problem so easily.
The only issue that you might run into if you're using a version before v2.2.0 is that
fab class conflicts with another class in Persona theme turning all brand icons into action buttons, but this is resolved in current versions of Persona.
If you have a paid version of FA5 it should be fairly simple to use it instead - you'd need to replace the FA files in
public/vendor/fontawesome with the new ones, set
_variables.less and import the light and duotone less files in
Also, I wanted to add that FA6 is slated to be added in NodeBB 3.0 (it's waiting until a major release since icon style being changed is possibly breaking) https://github.com/NodeBB/NodeBB/pull/10701