As explained on this page, PHP 8.0.0 expects a Shmop instance instead of a resource. According to bug #81098 this change means that the Shmop closes itself when the script completes so there is no need to close it with shmop_close.
Using function_exists('shmop_close') will return true but calling shmop_close will throw a deprecation error.
The documentation explains the change in 8.0.0 expects the expected argument changed without mentioning it'll just throw a deprecation error.