A few different ways. All involve a driver.
There are a few things you can drive: visibility, scale (scaling a mesh or bone to either 0 or 1), material transparency, ray visibility (in Cycles only). Your choice. Render visibility is the easiest thing to drive, but apparently viewport visibility cannot be driven, so it won’t preview properly.
To create a driver from a shapekey, right click on the value field of your shapekey and select “copy as new driver”. Then, right click on the property you wish to drive and select “paste as new driver”. Then open a new viewport, change it to a driver editor view, select the driven property, open up sidebar and select the drivers tab, and edit the driver as needed. How to edit it depends on what you’re driving.
For example, you might first change the driver to a “scripted expression” instead of a value, and then set the scripted expression to " value>0.9 " (but no quotes for you.) Then, anytime the driven value is greater than 0.9, the driven value will evaluate as “True”.