Despite the use of threadlocker being somewhat widespread (e.g. repair videos), I have yet to see a single recommendation to use it in any service manual. Perhaps this is my relative inexperience, but I thought I'd ask the community out of curiosity - do you use threadlocker? If so, what is your criteria for deciding which bolts receive threadlocker and which ones do not?
It depends on the vehicle but it's not uncommon to see threadlocker specified for assembly.
Use threadlocker when it's called for. Today many modern vehicles use one-time-only fasteners instead of threadlocker so it may be less common now than it was in the past.
I use threadlocker when the service manual says to, I replace fasteners when called for, and I just torque to spec when that is all that is specified.