I was wandering whether anybody else uses means outside of Fallout (or other games, even) to keep track of things within it?

I use a to-do app on my phone to track all the things that the Pip-Boy doesn't!

Things like:
All the mini missions that don't have 'quest-log' privileges
Tracking the code on the Boston Freedom Trail
Noting locations of locks/terminals that I don't yet have the skills for
Listing the buildings that I've 'discovered' but didn't have time to explore
Locations of particular difficulty that I'm not yet ready for
Guns/armour to buy when I have the money

I have separate lists for each character in every RPG I play, even non-RPG games often need it, I find!

Yep. After realizing that I had a list in my head of about 10 places I wanted to get back to, I started writing them down. The list is now over 50 locations and I've been back to, maybe, 26-27 of them. I cleared one more last night, but then found another to add to the list.

My list includes locations that don't show up as icons on the map (sneaky, Beth), like the little gem of a treasure trove I found in Boston.

I'm another who has made note of locks and terminals that were above my ability. Glad to say I've since been back to all of those.