first thing you have to make sure in order to use Switch you either need following ( again not sure if this is improved or alternate approach, i dont have exposure using Cosmos though)
py-mobile-controls Control which is part of UI kit ruleset as well as Pega-EnduserUI rulesets.
make sure you have above control included in your app skin - Additional included style tab. (Please check if override other controll or not, if yes then override iOS class from it as seperate custome style.
now next thing would be to use "iOS" css class in checkbox control property "Advance presentation options".
I think UI-Kit styling is different from Cosmos. The ios-slider class might be removed from the Cosmos Skin. Can you check what is the css applied with that class by inspecting page in the browser. You can appply the same css classes to your custom css file and include that in the skin if needed.
The slider control is nowhere to be found in Cosmos UI unlike in version 8.3 you can see it in the assignment list of the Review harness. Not sure if Pega is expecting us to deign custom controls in the Cosmos design system.