I built this the boring way. For about twenty trips I noted what I did not use, and removed it. What survived is short, and a few of the survivors are things I have never seen on anybody's list.
The principle that emerged
Almost everything is purchasable at the destination. The exceptions are things that are specific to you, things that are hard to find in a hurry, and things you would need urgently.
That principle removes most of what people pack. Toiletries, basic clothing, adaptors, umbrellas, and most gadgets are all available within walking distance of wherever you are staying, usually cheaply.
What it leaves is a much shorter list.
Clothing, ruthlessly
Three days of clothes regardless of trip length, and washing.
This took a while to accept and it is the largest single reduction available. Washing in a sink takes ten minutes and things dry overnight. Laundry services exist everywhere and are inexpensive.
Everything in a small number of colours so that anything works with anything, which eliminates the outfit-planning that drives most overpacking.
One item warmer than you expect to need, because being cold is miserable and buying a jumper in a hurry is expensive.
And shoes: the ones on your feet plus one other pair, at most. Shoes are the heaviest thing anybody packs and the least used.
The things nobody lists
A metre of gaffer tape, wrapped around a card. It has fixed a bag strap, a shoe, a broken zip, a rattling window and a cable. It weighs nothing.
A cheap doorstop. Sounds paranoid, weighs almost nothing, and makes a room with a questionable lock into a room you can sleep in.
Earplugs and a proper eye mask. The single highest return per gram of anything I carry, given how much of travelling is being tired in rooms that are too bright and too loud.
A folded fabric bag, the kind that packs into itself. Used constantly for markets, for laundry, and for the day when you buy more than you can carry.
A pen. Immigration forms, and the queue for a shared pen at the back of an aircraft.
Documents and money
Photographs of the passport, visas, insurance policy and any bookings, stored somewhere accessible without a phone, plus a printed copy in the bag.
This has mattered twice. Both times the phone was the problem and the printed copy was what worked.
Two payment cards from different banks, kept in different places. Cards get blocked, and a single card is a single point of failure in a country where you may have limited options.
Enough cash in a widely accepted currency to get out of a problem, kept separately from everything else.
The medical bit
Whatever you take regularly, in its original packaging with the prescription, in the bag you carry rather than in checked luggage. This is not optional and people get it wrong constantly.
Enough for the trip plus several extra days, because delays happen and pharmacies abroad may not stock your specific version.
And a very small kit: painkillers, something for an upset stomach, blister plasters, and any antihistamine you rely on. Everything else can be bought.
Electronics, minimised
One charger capable of charging everything, which now exists and which removes four cables from the bag.
A short extension with several sockets, which turns one adaptor into four and solves the standard problem of a hotel room with two accessible sockets both behind the bed.
A battery pack, sized to what you actually need rather than the largest available. Mine is small and it has never run out.
And that is genuinely all. The camera goes in if the trip is for photographs and otherwise stays at home, which was a difficult admission.
The bag itself
Carry-on sized, always, even for long trips.
The reasons are practical rather than principled. No waiting, no loss, no fee, and the freedom to change plans at an airport without retrieving anything.
It also enforces the rest of the list, which is the real function. A bag that cannot hold more than you need is more effective than any amount of discipline.
What I got wrong for years
Packing for contingencies that never happened, which is where almost all excess weight comes from.
The formal clothes for an occasion that did not arise. The equipment for an activity I did not do. The books I did not read.
The honest question for each item is not whether you might use it. It is what happens if you need it and do not have it, and for nearly everything the answer is that you buy one.
The bag inside the bag
One organisational thing that has made more difference than any individual item.
Everything goes into a small number of labelled pouches rather than loose. Clothes in one, electronics in one, documents and medical in one.
The benefit is not tidiness, it is that packing to leave takes four minutes instead of twenty, and that nothing gets left in a drawer, which is how I have lost things repeatedly.
It also means a security check does not turn into an unpacking exercise, since the pouch comes out and goes back as a unit.