I have 52K miles on a 2013 S40.
all but the first 1100 are mine. It is flawlessly maintained and runs like new, except very recently.

Recently - perhaps since the rectifier quit? - burning out the headlight.. and was immediately replaced - the engine knocks and pings badly at Wide Open Throttle, like while climbing a mountain grade at 65, or accelerating hard out of a curve.

Yesterday I was forced to buy 87 octane fuel in the middle of nowhere  (I otherwise always run 92 ethanol free) and the knocking/pinging was awful.  

Symptoms quickly resolve when throttle is reduced.

Where should I start?  Is spark advance somehow vulnerable?  Might the main jet be clogged?

Congrats on 52k miles!  At that mileage, and it only occurring at higher rpms under a load, I’d guess carbon buildup in the cylinder.  I’d inspect the spark plug and look for clues.  If you have an endoscopic camera, try to get a look inside the cylinder through the spark plug hole.

Since you’ve been doing regular maintenance and using quality fuel, and you haven’t had this issue before, I doubt it’s a dirty main jet.  The primary jet would clog and show issues before the main jet would (usually).  But…check the air filter….maybe it could be that simple.  Or your bike has become spoiled on high octane 92 and let you know it when you filled up with 87 octane. I’ve always just used 87 top tier gas and haven’t had any knocking/pinging (stock engine, no high comp piston)

I’m no mechanical guru, but a fresh spark plug and new air filter have done wonders for me in the past.  And it’s cheap and easy…and it never hurts to have a fresh plug and air filter if you have to dig into in further (at least you can rule those out).  Hopefully you don’t have to pull the head and clean it and the valves and piston.  Because I’m lazy and not a gear head, I’d be inclined to try some Seafoam or Berryman’s Chem Tool (high mileage) before opening up the engine.

Summary - New spark plug, air filter, and Berryman’s for starters.

Berryman’s comes in 2 flavors; B12 and B60 (high mileage).  I’d go with B60 in the gas tank.  B12 did a really good job on a piston I cleaned once removed from the engine. I haven’t tried the “Total Fuel System Treatment” product (maybe that’s flavor #3), but it specifically lists combustion chamber.

From Berryman’s website… —>
https://www.berrymanproducts.com/question/can-i-use-b60-like-b12/#:~:text=It's%20true%20that%20B%2D60,B%2D12%20for%20your%20soaks.