Marshmallow Heads-up Notifications Gone?
The update never removed Heads Up notifications. You just have to select which apps have the permission to display them.

Go into Settings -&gt; Sound &amp; notification -&gt; App notifications to pull up a list of apps.

Once you select Whatsapp in your case, there will be an "Allow Peeking" checkbox, as so:


(source: staticworld.net)  

Just tick that back on, and you'll get those notifications.



If WhatsApp doesn't respect those settings, there is an alternative:


You can download a Fully Customizable Heads-up for WhatsApp from Play Store, which works with the latest WhatsApp update.




Hope this helps!