WordPress Issues and Workarounds After Upgrading to 8.1

This workaround only applies if you use the same theme and plugin as I do. It only hides the error messages without affecting the frontend display; it does not fix the underlying issue. We still need to wait for a fix from the developers.
Theme: Neve 3.7.2
Plugin: WP Super Cache 1.10.0

Post editor: Warning: Trying to access array offset on value of type bool in /www/wwwroot/xxx.xxx.net/wp-content/themes/neve/inc/views/template_parts.php on line 432

This PHP error report says that line 432 of template_parts.php tries to access an array offset on a boolean variable, which is not allowed.

In programming, a boolean has only two possible values: true or false. It cannot store multiple values like an array. Trying to access an array offset on a boolean therefore causes an error.

Change from

$markup  = '';
        $markup .= '’;

To

if (is_array($read_more_args) && isset($read_more_args['classes'], $read_more_args['text'])) {
    $markup  = '';
    $markup .= '';
} else {
    // Handle the error case here.
}

Post editor: Warning: Undefined variable $markup in /www/wwwroot/xxx.net/wp-content/themes/neve/inc/views/template_parts.php on line 453

Change from

if ( ! empty( $read_more_args['classes'] ) ) {
            $markup = '
' . $markup . '';
        }

        $new_moretag .= $markup;

        return $new_moretag;

To

$markup = isset($markup) ? $markup : '';

if ( ! empty( $read_more_args['classes'] ) ) {
    $markup = '
' . $markup . '';
}

$new_moretag .= $markup;

return $new_moretag;

Page displays: Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in /www/wwwroot/xxx.net/wp-content/plugins/wp-super-cache/wp-cache-phase2.php on line 54

Change from

return do_cacheaction(
        'wp_cache_key',
        // phpcs:ignore WordPress.NamingConventions.ValidVariableName.VariableNotSnakeCase
        wp_cache_check_mobile( $WPSC_HTTP_HOST . $server_port . preg_replace( '/#.*$/', '', str_replace( '/index.php', '/', $url ) ) . $wp_cache_gzip_encoding . wp_cache_get_cookies_values() . '-' . wpsc_get_accept_header() . $accept_tag )
    );

To

$url = $url ?? '';
return do_cacheaction(
    'wp_cache_key',
    wp_cache_check_mobile( $WPSC_HTTP_HOST . $server_port . preg_replace( '/#.*$/', '', str_replace( '/index.php', '/', $url ) ) . $wp_cache_gzip_encoding . wp_cache_get_cookies_values() . '-' . wpsc_get_accept_header() . $accept_tag )
);

Page displays: Deprecated: parse_str(): Passing null to parameter #1 ($string) of type string is deprecated in /www/wwwroot/xxx.net/wp-content/themes/neve/inc/views/pluggable/pagination.php on line 198

Change from

parse_str( wp_parse_url( get_pagenum_link(), PHP_URL_QUERY ), $url_query_args );

To

$query_str = wp_parse_url( get_pagenum_link(), PHP_URL_QUERY );
$query_str = $query_str ?? '';
parse_str($query_str, $url_query_args);