Steps to reproduce the issue

to reproduce with a core component (com_contact)

On j3 site add the following function to ./components/com_contact/controllers/contact.php

public function test()
{
	var_dump($this->input->getArray());die;
}

On J4 site add the same method in ./components/com_contact/src/controller/ContactController.php

Now lets call this method on both the J3 and J4 site:

[J3domain]/index.php?option=com_contact&task=contact.test
[J4domain]/index.php?option=com_contact&task=contact.test

Actual result on J3 site

array(5) {
  ["option"]=>
  string(11) "com_contact"
  ["task"]=>
  string(4) "test"
  ["language"]=>
  string(5) "en-GB"
  ["Itemid"]=>
  NULL
  ["lang"]=>
  string(5) "en-GB"
}

Actual result on J4 site

array(7) {
  ["option"]=>
  string(11) "com_contact"
  ["task"]=>
  string(4) "test"
  ["language"]=>
  string(5) "en-GB"
  ["view"]=>
  string(7) "article"
  ["id"]=>
  string(1) "8"
  ["Itemid"]=>
  int(1142)
  ["controller"]=>
  string(7) "contact"
}

System information (as much as possible)

J3.10.11 and J4.2.4

Additional comments

Been troubleshooting this on several sites / extensions and found that this is (IMO) a B/C break.
I think this is due to J4 adding the Itemid without that being called for. In the example given here it adds Itemid 1142 (which is my frontpage, com_content). This is not the issue,: the issue is that it also adds the view and when not specified on the url the id.

So if the extension reads if the id is present ($this->input->get('id', '')) and acts upon it, then in J4 it will always act upon it: not in this case on the contact id, but on the article id (8). So the id is not for the contact but for the article leading to unwanted behavior.