The variable is now called
org-babel-default-header-args. There are also similar variables for each language supported by babel. I don't remember what
params meant in older versions, however, so I'm not sure if this is an exact replacement.
C-h-v org-babel- TAB is a nice way to find out all the variables whose names have the prefix
org-babel-. And one thing that org 9.x has going for it is that it has much more predictable names for variables and functions in general.
EDIT: I read your comment and took a look through the code: I don't think that what you want can be done with unmodified org-mode. I was only able to do it by adding a (dynamically bound) variable and assigning to it the (internal) value of params just before the block was executed. Obviously, that invalidates your warranty :-). Here's the diff:
diff --git a/lisp/ob-core.el b/lisp/ob-core.el
index 9f67000..dfcc7a9 100644
@@ -628,6 +628,8 @@ a list with the following pattern:
(setf (nth 2 info) (org-babel-generate-file-param name (nth 2 info)))
(defun org-babel-execute-src-block (&optional arg info params)
"Execute the current source code block.
@@ -696,6 +698,7 @@ block."
(let ((name (nth 4 info)))
(if name (format " (%s)" name) "")))
+ (setq org-babel-params params)
(if (member "none" result-params)
(progn (funcall cmd body params)
(message "result silenced"))
The source block would be modified to print