The template I’m using outputs the date of a post using:

echo get_the_date( get_option( 'date_time' ) );

I was wondering why this output is now (after upgrade to WP 5.5 and PHP 7.4) not working anymore?

A fast fix is using the option date_format:

echo get_the_date( get_option( 'date_format' ) );

I looked into WordPress Developer https://developer.wordpress.org/reference/functions/get_the_date/#source looks like it suppose also to work when the option ‘date_time’ is empty:

function get_the_date( $format = '', $post = null ) {
    $post = get_post( $post );
 
    if ( ! $post ) {
        return false;
    }
 
    if ( '' === $format ) { // <-- check for empty value
        $the_date = get_post_time( get_option( 'date_format' ), false, $post, true );
    } else {
        $the_date = get_post_time( $format, false, $post, true );
    }
 
    return apply_filters( 'get_the_date', $the_date, $format, $post );
}

Why does the output of get_the_date(get_option('date_time')) in the template doesnt work with WordPress 5.5 (PHP 7.4)?