PHP error: Trying to access array offset on value of type bool

Whoops – sorry about the omissions.
WP 5.6
PHP 7.4

You ask “which rename feature?” Under the Brute Force section, I have Enable Rename Login Page checked and it sets my renamed login page. I do not see any other features on that page. I have been using that for a long time.
I do not have the Cookie Based Brute Force Login Prevention enabled.

I regret that I cannot say specifically what changed to cause this. I updated several plugins, etc. and maybe PHP before spotting it.
However, I filter login_message to embellish what the user sees when requesting a password reset email. This code started to give errors with something like action not an array element. I fixed it by testing for the existence of the key first. So in this code, I had to insert the array_key_exists condition.

// (M)	Add additional advice to the Lost Password page
function ovni_embellish_retrieve_password_message($message){
	if (array_key_exists('action', $_REQUEST)){
		if (($action =  $_REQUEST['action' ]) == "lostpassword"){
			$message_addition = " If you don't see the email, check your spam filter.<br>If you repeat your request for a reset link, note the time when you make the request. Use the link in the email sent at that time. The link in earlier emails will no longer be valid.</p>";
			$message = substr_replace($message,$message_addition,-4);
			}
		}
	return $message;
	}
add_filter('login_message', 'ovni_embellish_retrieve_password_message', 1);

I think the issue I am seeing with AIOWP handling a redirection of the login page is related. And I think the other non-AIOWP error I mentioned in my original post arises from a similar issue.

IMHO I think code needs to handle invalid indexes gracefully. Perhaps recent updates to PHP have made it less tolerant of them?
I hope that helps.