2 Examples on how to change variation SKU separators w/ Product SKU Generator

wc-sku-generator-replace-variation-separator.php

<?php // only copy this line if needed

/**
 * Example 1: Changes the separator between variations in a generated SKU
 *
 * @param string $separator the original separator
 * @return string the new separator
 */
function sv_wc_replace_variation_sku_separator( $separator ) {
	return '_';
}
add_filter( 'wc_sku_generator_attribute_separator', 'sv_wc_replace_variation_sku_separator' );


// OR


// Example 2: Removes the separator between variations in a generated SKU
add_filter( 'wc_sku_generator_attribute_separator', '__return_empty_string' );