And as far as I know, ways to "communicate with the outside world" are not commonly taken on hikes even these days.I don't think flares would even be practical because they are only visible from far away at night, and are likely to be missed.A satellite phone would be a better option and is the same price as a smartphone, but I haven't heard about many hikers taking satellite phones with them either...
Where I live (Yukon), lots of big hikers have satellite phones (or satellite GPS with tracking and SOS option), and it's pretty easy to rent them from touring business is you need one just for one back-country, off-trail trip.
A modern flare (the one they use for boat safety) can be seen from 40km, and are burning for 40 seconds. It's not a long distance (the DPI is more than 40km away from district 41) , and not a long time... In the DPI context, it makes no sens to carry a one-use (pretty heavy, explosive, maybe humidity-sensible) short-lived signal that would not be seen from far enough -and only at night.
(especially since it would take quite more than one to lead some help to where there are needed : after just one (seen) flare, the search zone would still be immense !)
A radio have the same problem : only some bigger ones could have send SOS across the distance and it was too heavy to carry on a trail. Still : it would have made more sens than flares to me.
What people used to carry a few years ago as "emergency signal" was... a whistle.
It can be heard from 10km (with big variations in mountains, the sound can carry quite further or quite shorter than that), but you can blow it every minute. If you are lucky enough to be heard, your whistling will drive them right to you quickly.