var days = mutableStateListOf(
        Cell("Sun", false),
        Cell("Mon", false),
        Cell("Tue", false),
        Cell("Wed", false),
        Cell("Thu", false),
        Cell("Fri", false),
        Cell("Sat", false)
    )

fun changeDaySelection(index: Int) {
//        days[index].isSelected = !days[index].isSelected <- this line does cause recomposition 
        days[index] = Cell(days[index].text, !days[index].isSelected) // <- this line does cause recomposition
    }

var isSelected: State<Boolean>

data class Cell(..., val isSelected: Boolean)
// note the val

var days by remember { mutableStateListOf(...) }
...
days = days[index].copy(isSelected = !days[index].isSelected)

=