I actually just took mine off for the first time last weekend, with mileage at 23,000 miles. I ride in all weather, and the bike is stored outside all year usually. I often get milk eventually running out of the vent at least every 3,000 miles or sooner. As expected, when I popped the swing arm off, water just poured out. Low to medium amount of rust on the driveshaft. Splines didn’t seem bad, including normal wear. I cleaned and greased (and dried) everything, and sealed up the boot.
I noticed that the edge of the boot at the end where the inner ring presses is not even, leaving very little rubber to extend past the ring in one spot, so I siliconed both ends of the boot. Just finished replacing the final drive oil, so we’ll see.