It’s less about the aftermarket and more about OEM kit.
At the moment 10speed SLX vs 11speed XT provides a nice clean differentiator between mid and higher price point complete bikes…
So the OEMs would probably like to hold SLX 11speed back until at least this winter, as it helps make those XT spec’d off the peg bikes that bit more desirable…
looking at “Dynasis” and clutched mechs as a recent examples, the filter down from XTR, to XT and then all the way down to Deore only seems to take a couple of years, shimano know that low-mid price point matters more in terms of market share. So rapid filter down is more in their interest… I reckon the rumours and “spy shots” will be deployed in the next couple of months, even if you can’t buy it before October…