Is there a reason that the ST1100 rear fender doesn’t extend down past the swing arm?
Anything in contact with the swing-arm (and road grime added as
the perfect abrasive) will rub and grind, sanding through paint layers, primer, zinc plating, eventually cutting into the steel...
so most likely just accelerating corrosion on the contact patches created...
The stiffening transverse box and sleeving do feature draining holes on the bottom to prevent water accumulating limiting the unseen corrosion caused by it; over decades they get clogged with road grime...
frequent rising, cleaning and cavity protection does help...
Its normally not even a real problem for most riders, especially those in zones with either no winter at all, or those with real winters, where snow and ice covered roads simply inhibits any riding...
But for some odd reasons you Brits seem to simply enjoy to slip and slide around in the highly concentrated brine of your winter roads, whilst at the same time obviously neglect frequent washing to at least dilute if not remove the salt accumulated... leading the issue shown in your picture there...
Frequent cleaning and conserving the cavities by fogging them with ACF-50 or Waxoyl might not have completely eliminated the problem, but at least have helped slowing it down significantly...
I'd rather ground the ST until you get hold of a decent replacement swing-arm (eBay), over the risk of the corroded transverse box collapsing... MHO...