Confirmed for J!3.4.6, PHP5.6.13

In my case it was a plugin of mine. My fault.
A call like this

$ID = (int) $this->app->input->get('id');

creates the notice because id is an array in com_tags/tag view context.

After removing this line the notice was gone. So, I think it's not a Joomla issue.

(And I remember now that we had this already in 2 or 3 forum threads. Always 3rd extensions.)

I have disabled all the "content" plugins (both Joomla and mine) and I still get the notice. I suspect that it has something to do with Turkish characters.

What type was your plugin and what plugin was it?

And the same notice disappears if the URLs are not search engine friendly. How would you explain this?

It's a "private" system plugin in method onAfterRoute.

SEF off: It makes no difference. Still get the notice from my plugin.
That makes sense (id[0]=25):
URL:

index.php?option=com_tags&view=tag&id[0]=25&Itemid=650

At the moment I use a quick fix and handle id always as array.

$ID = $this->app->input->get('id', array(), 'ARRAY');

... do more...

You can try debugging in
/libraries/joomla/input/input.php method get(...)

 public function get($name, $default = null, $filter = 'cmd')
 {
  if (isset($this->data[$name]))
  {
#DEBUG START
if ($filter == 'cmd' && is_array($this->data[$name]))
{
 $callers = debug_backtrace();
 echo 'DEBUG: '.print_r($callers[0],true);exit;
}
#DEBUG END
   return $this->filter->clean($this->data[$name], $filter);
  }

  return $default;
 }

Further discussions should be done in a forum I think.

@bertmert very big thank you for your help and for your time. You were right, it was because of a non Joomla plugin. The thing is, I have never seen this notice before, especially before the update.