The popular WordPress plugin W3 TotalCache appends a comment in the footer like this:
<!--
Performance optimized by W3 Total Cache. Learn more: https://www.boldgrid.com/w3-total-cache/
Object Caching 51/56 objects using memcached
Page Caching using memcached
Lazy Loading (feed)
Minified using memcached
Database Caching 12/27 queries in 0.011 seconds using memcached
Fragment Caching 0/0 fragments using memcached
Served from: gbe0.com @ 2022-03-06 13:21:13 by W3 Total Cache
-->
Add the following line to a site plugin or wp-config.php:
// Disable W3 TotalCache footer comment
add_filter( 'w3tc_can_print_comment', function( $w3tc_setting ) { return false; }, 10, 1 );
Note that this will impact the functionality to print cache statistics; the line can be commented temporary while those are enabled if required.