The comment_text_rss() WordPress PHP function is used to display the current comment content for use in the feeds. It takes no parameters.
Usage
To use comment_text_rss(), simply call it within the loop in your RSS template. The function will automatically output the comment content, preformatted for RSS use.
while ( have_comments() ) : the_comment();
    comment_text_rss();
endwhile;
In the above example, it will display all comments for a post in an RSS format.
Parameters
- None
More information
See WordPress Developer Resources: comment_text_rss()
This function is implemented in WordPress 1.5.0 and is not yet depreciated. The source code for the function can be found in wp-includes/comment-template.php. Related functions include get_comment_text(), comment_text() and the_comment(). 
Examples
Displaying comments in RSS for a specific post
You can use the comment_text_rss() function to display all comments for a specific post in an RSS format.
// Get comments for a post.
$comments = get_comments( array(
    'post_id' => $post_id,
    'status' => 'approve'
) );
// Loop through each comment.
foreach ( $comments as $comment ) {
    // Set the global comment variable.
    $GLOBALS['comment'] = $comment;
    // Display the comment text.
    comment_text_rss();
}
Displaying comments with a custom wrapper
You can customize the output of comment_text_rss() by wrapping it in custom HTML.
while ( have_comments() ) : the_comment();
    echo '<div class="comment-rss">';
    comment_text_rss();
    echo '</div>';
endwhile;
Trimming comment output
You can limit the amount of text that comment_text_rss() outputs by using the wp_trim_words() function.
while ( have_comments() ) : the_comment();
    $comment_rss = get_comment_text();
    echo wp_trim_words( $comment_rss, 20, '...' );
endwhile;
Combining comment_text_rss() with comment_author_rss()
Display the comment author’s name along with the comment text.
while ( have_comments() ) : the_comment();
    comment_author_rss();
    echo ': ';
    comment_text_rss();
endwhile;
Combining comment_text_rss() with comment_date_rss()
Display the comment date along with the comment text.
while ( have_comments() ) : the_comment();
    comment_date_rss();
    echo ': ';
    comment_text_rss();
endwhile;
Remember, comment_text_rss() should be used within the loop in your RSS template to ensure it has access to the correct comment data.