A question mark before the parameter method this is a bug in PHP?

Types for parameters and return values can be marked as nullable by adding a prefix in the form of a question mark. This means that the parameters and return values can be specified type or NULL.

\rphp.net/manual/ru/migration71.new-features.php

No, it means the same as ($par=null)
In other words - an optional parameter.

The question mark means that the parameter can accept the specified type or null

In other words - an optional parameter.

Actually - no.
The question mark indicates that the parameter must be either null or the specified type. But let him not

function test (?stdClass $obj) { // some code } test(new stdClass); // ok test(null); //ok test(); // error

In the example in the question parameter is really optional, as indicated by the default value.