I just had to search the longest time for the boolean property. I actually ended up switching back to the old UI to create the property and upon more research and talking to team members, I found where the boolean properties are hidden.
First, there is no tool tip when hovering over the highlighted item, so no explanation on what this button does, even though every other item in the appearance section has a tool tip.
Upon pressing the unlabeled button, I get a list of variables. The + in this view, I would assume creates a new variable, since I am in a variable window.
Finally, I found that clicking that + button actually creates a boolean property?? I have no idea why this is paired with creating a variable. Why is the boolean property not clearly surfaces like variants, text, or instance properties are? I suggest surfacing this property, like it originally did on the old UI to create a faster and more intuitive work flow.