bbPress.org

it’s in

/includes/forums/template.php

lines

1987, 1995, 2007, 2017

You could probably filter it by

//This function changes the text wherever it is quoted
function change_translate_text( $translated_text ) {
	if ( $translated_text == 'This forum contains %1$s and %2$s, and was last updated by %3$s %4$s.' ) {
	$translated_text = 'This elephant contains %1$s and %2$s, and was last giraffed by %3$s %4$s.';
	}
if ( $translated_text == 'This forum contains %1$s, and was last updated by %2$s %3$s.' ) {
	$translated_text = 'This elephant contains %1$s, and was last giraffed by %2$s %3$s.';
	}
if ( $translated_text == 'This forum contains %1$s and %2$s.' ) {
	$translated_text = 'This elephant contains %1$s and %2$s.';
	}
if ( $translated_text == 'This forum contains %1$s.' ) {
	$translated_text = 'This elephant contains %1$s.';
	}
	return $translated_text;
}
add_filter( 'gettext', 'change_translate_text', 20 );

in your functions file

Functions files and child themes – explained !