How to access the thank you page from the single post?

^{Note: WordPress deliberately misspelled the word ‘referer’, and thus, so did I..}

If you just want to check if the referer is your contact-us Page or any single Post page, then you can first retrieve the current URL path (i.e. example.com/<this part>/) and then check if the path is exactly contact-us (or whatever is your contact Page’s slug), and use get_posts() to find a single post having that path as the slug.

So try this, which worked for me (tested with WordPress 5.6.1):

// Replace this:
if (wp_get_referer() === get_site_url().'/contact-us/') {
    return;
}

// with this:
$path = trim( parse_url( wp_get_referer(), PHP_URL_PATH ), "https://wordpress.stackexchange.com/" );
// If it's the contact page, don't redirect.
if ( 'contact-us' === $path ) {
    return;
} elseif ( $path ) {
    $posts = get_posts( array(
        'name'           => $path,
        'posts_per_page' => 1,
    ) );
    // If a post with the slug in $path was found, don't redirect.
    if ( ! empty( $posts ) ) {
        return;
    }
}

• But note that the code would only work if your permalink structure (for the default post type) is /%postname%/ — with or without the leading and/or trailing slashes.

And bear in mind that the referer can be easily spoofed, so you might want to consider a more reliable approach like using transients or nonces with a short lifespan.. 🙂