Firstly the environment (release 2.4.0 gives the same, updated to nightly with the hope it will work there):
wp --info
OS: Linux 5.4.0-28-generic #32-Ubuntu SMP Wed Apr 22 17:40:10 UTC 2020 x86_64
Shell: /bin/bash
PHP binary: /usr/bin/php8.0
PHP version: 8.0.3
php.ini used: /etc/php/8.0/cli/php.ini
MySQL binary: /usr/bin/mysql
MySQL version: mysql Ver 15.1 Distrib 10.3.25-MariaDB, for debian-linux-gnu (x86_64) using readline 5.2
SQL modes: STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
WP-CLI root dir: phar://wp-cli.phar/vendor/wp-cli/wp-cli
WP-CLI vendor dir: phar://wp-cli.phar/vendor
WP_CLI phar path: /var/www/swellsyme.com
WP-CLI packages dir:
WP-CLI global config:
WP-CLI project config:
WP-CLI version: 2.5.0-alpha-3f4c34b

The error:

sudo -u www-data -- wp core download
PHP Fatal error: Uncaught TypeError: implode(): Argument #2 ($array) must be of type ?array, string given in phar:///usr/local/bin/wp/vendor/rmccue/requests/library/Requests/Transport/fsockopen.php:195
Stack trace:
#0 phar:///usr/local/bin/wp/vendor/rmccue/requests/library/Requests/Transport/fsockopen.php(195): implode()
#1 phar:///usr/local/bin/wp/vendor/rmccue/requests/library/Requests.php(379): Requests_Transport_fsockopen->request()
#2 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/utils.php(775): Requests::request()
#3 phar:///usr/local/bin/wp/vendor/wp-cli/core-command/src/Core_Command.php(310): WP_CLI\Utils\http_request()
#4 phar:///usr/local/bin/wp/vendor/wp-cli/core-command/src/Core_Command.php(319): Core_Command::read()
#5 phar:///usr/local/bin/wp/vendor/wp-cli/core-command/src/Core_Command.php(183): Core_Command->get_download_offer()
#6 [internal function]: Core_Command->download()
#7 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Dispatcher/CommandFactory.php(99): call_user_func()
#8 [internal function]: WP_CLI\Dispatcher\CommandFactory::WP_CLI\Dispatcher{closure}()
#9 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Dispatcher/Subcommand.php(489): call_user_func()
#10 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Runner.php(380): WP_CLI\Dispatcher\Subcommand->invoke()
#11 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Runner.php(403): WP_CLI\Runner->run_command()
#12 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Runner.php(87): WP_CLI\Runner->run_command_and_exit()
#13 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Runner.php(1116): WP_CLI\Runner->do_early_invoke()
#14 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/WP_CLI/Bootstrap/LaunchRunner.php(23): WP_CLI\Runner->start()
#15 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/bootstrap.php(75): WP_CLI\Bootstrap\LaunchRunner->process()
#16 phar:///usr/local/bin/wp/vendor/wp-cli/wp-cli/php/wp-cli.php(27): WP_CLI\bootstrap()
#17 phar:///usr/local/bin/wp/php/boot-phar.php(11): include('...')
#18 /usr/local/bin/wp(4): include('...')
#19 {main}
thrown in phar:///usr/local/bin/wp/vendor/rmccue/requests/library/Requests/Transport/fsockopen.php on line 195