I would expect the spline in either the drive hub or the driveshaft to make considerable noise if they are "sliding" past each other
But with as much slipping of the clutch as you have had- I would expect by now to see hot spots on the clutch steels and the frictions will be well burnished and need replacement
But that kind of "yuk" in your master... well I have not seen it cause clutch slipping- usually I have seen that cause more issues with trying to get the clutch to disengage. I have seen dozens of bikes come through with fluid that has a jello consistency of all sorts of colors and they didn't make the clutch slip
And with fluid that neglected, look at the bushing and the pivot bolt too--clean and lube them.
we have seen almost a dozen bad rear wheel hubs in the last 5 years, and none of them made a sound, and were only discovered by careful analysis.
so, it doesn't have to be audible to diagnose... last one was not many months ago.
as for the master cylinder, the small holes in the reservoir can block easily, and o/p noted gack in there also... if the return is blocked, the heat of the engine will pressurize the lines, causing partial disengagement, and slippage also...
I say this from a few years experience this bike, diagnosing issues. mostly from afar.