that's a pretty interesting statement.
There's definitely a way to report on which reports a custom field appears on (in a parameter report just pull in the Reports column). At the very least you can use this functionality, and export to a spreadsheet and sort or filter additionally by this column.
However, you should know that in other instances, just because we don't put a field ***on a report*** doesn't mean it's not being used. (as a very broad example, section breaks and descriptive text fields would fall into this category, but there are plenty of regular custom fields we choose not to report on)