As it says in the subject. As an example, from Poll.php
Functional\poll(): Implicitly marking parameter $delaySequence as nullable is
deprecated, the explicit nullable type must be used instead in
vendor/lstrojny/functional-php/src/Functional/Poll.php on line 29.
I should be able to put together a pull request some time this weekend, as soon as I identify the other PHP 8.x issues in my project.