An apps script to detect when a Category is updated in a google spreadsheet, to update the subcategory data validation.

subcategory-validation.gs

// When editing a cell that has data validation of the named range "CategoryList"
// The cell to the right of it will have it's data validation set to the name range of category + "List"

function onEdit(e) {
  // Logger.log('start');
  // Go see what was edited. Grab the top left cell in case they pasted both category and subcategory
  const edited = e.range.offset(0, 0, 1, 1);
  const spreadsheet = edited.getSheet().getParent();
  // See if it has a validation
  const validation = edited.getDataValidation();
  if (validation == null) { 
    return;
  }
  // See if it points to a range
  if (validation.getCriteriaType() != SpreadsheetApp.DataValidationCriteria.VALUE_IN_RANGE) {
    return;
  }
  // Go get that range
  const validationRange = validation.getCriteriaValues()[0];

  // Go find if that range is based on a named range
  const namedRange = spreadsheet.getNamedRanges().find((namedRange) => {
    // Logger.log('test: %s %s', namedRange.getRange().getA1Notation(), validationRange.getA1Notation());
    return namedRange.getRange().getA1Notation() === validationRange.getA1Notation()
  });

  // We couldn't find a named range
  if (namedRange == null) {
    // Logger.log('could not find parent');
    return;
  }

  // We only care about the categories range
  if (namedRange.getName() !== 'CategoriesList') {
    // Logger.log('not the right name');
    return;
  }

  // Get the subcategory
  const childCell = edited.offset(0, 1);
  const subNamedRangeName = edited.getValue() + 'List';
  const subNamedRange = spreadsheet.getRangeByName(subNamedRangeName);

  // If the value is not a named range, then we can't do anything.
  if (subNamedRange != null) {
    // At this point we modified a cell who's data validation is the parent categories list.
    // Now we need to set the child validation range. We'll assume the cell to the right is the spot to do it.
    childCell.setDataValidation(SpreadsheetApp.newDataValidation()
      .requireValueInRange(subNamedRange)
      .setAllowInvalid(false)
      .build()
    );
  } else {
    // The validation isn't valid anymore (perhaps because a value was deleted)
    // Remove the child validation
    childCell.clearDataValidations();
  }
}

function backfill() {
  const backfillRange = SpreadsheetApp.getActiveSpreadsheet().getRange('K4:K48');
  for (let row = 0; row < backfillRange.getNumRows(); row++) {
      const backfillCell = backfillRange.offset(row, 0, 1, 1);
      onEdit({range: backfillCell});
    }
}