After discussing this with the WP plugin review team.
The precommit hook that is being run on the server does not yet support PHP8.
Removing any dependencies on the PHP80 polyfill code in vendor packages should fix this until they upgrade the precommit hooks to support PHP8.
Just noting on the discussion above related to symfony/polyfill-mbstring the Symfony polyfills now include files which are only PHP8-syntax, but those files are only included on PHP8, the PHP7 variant is included for the rest of the users.
Same situation as when the libraries added PHP 7.3/7.4 syntax and we were strictly requiring 7.0 or something.
Using the older version of the library is the proper solution for now.
The SVN server will lint with PHP8 once it’s deployed for usage on WordPress.org web requests, which while it has no timeline, I expect it’s probably going to happen at some point in the next few months, but before it’s widespread adoption.