Use the concatenation operator
},"select * where Col1 <>''"
&IF(FALSE," order by Col24 DESC",))
The formula syntax doesn't include something similar to the PHP
/* */ multiple line comment or HTML
<!-- -->, but you could use functions like
IF. Another alternative is
IFERROR. If the formula returns a number,
N could be another alternative, as it returns
0 when it's argument isn't a number.
In this case I suggest the use of
IF because the
QUERY's second argument should be a text value.
& concatenates the two operands and returns a text value; by using
IF we could set a condition and the value to be returned if the condition is true, in this case
order by Col24 DESC (please note the leading space), and another value if the condition is false, in this case I suggest to leave the second argument blank, to return a blank.
You could go even further by using a cell reference for the condition to toggle the inclusion/exclusion of the order clause.
Pro-tip: Use Google Apps Script to document and toggle formulas. A cell note or comment could be used to mark those formulas that have being documented this way.