Yep, Netflix did remove "Deadliest Warrior" from its online streaming service because the show was no longer in high demand. Only series' that people are constantly requesting are kept as "available to watch", otherwise, shows are removed and added to the rent list.
So if you are in the United States (like Joao said above) then you can do Netflix the old fashioned way and order the DVD which will come in the mail to your address on file. Then you can get comfy on your couch and watch "Deadliest Warrior" as you wish (lucky you). Netflix has all three seasons for rent.
Where can you watch Deadliest Warrior for free online?
Not to worry if you don't live in America or don't want to order the hard copy DVD. There lots of places online where you can stream "Deadliest Warrior" (and many other top and little known TV series') online.
Watchseries.to has all three seasons of your show available for free streaming from multiple links :)
Lots of "Deadliest Warrior" episodes are also on Youtube in full. Here's the battle between Genghis Khan, the great Mongolian warrior and Hannibal of North Africa, enemy of Rome.
Here's another episode: Joan of Arc vs. William the Conqueror