When creating a true/false field there is a selection for “Stylised UI’. This is supposed to default to No and any existing true/false field should be defaulting to No and showing the checkbox and not the slider. I tested this on a site, created a true false field in acf 5.4 and then upgraded to acf 5.5. My true false field remained a checkbox. So this is something you should have to turn on and not the default behavior.