Landsknecht has always been depicted wearing if not breastplate, no armor at all. Though their strength is in their speed that comes with their size and formation, individually they are at risk of quick death without armor protecting parts of their body. So how do they cover this shortcoming? Do they rely solely on their attacking capabilities? Were their formations the only protection they could rely on? Did their extravagant clothing add some layer of protection?
Well, you're basinh all of this on misunderstanding. The landsknechte absolutely wore armor, in fact they were generally comparatively heavily armored for infantry units of the time. Common surviving examples of landsknecht armor tend to be munitions grade half-plate harnesses, sometimes refered to as almain rivet in england. Cheaper than for example white armor or demi-lancer armor but still great protection for a reasonable price, especially compared to what was generally available to levies or militias at the time. Breastplates with faulds and tassets are also fairly common.
However, Landsknechte were mercenaries and as such, each soldier was responsible for procuring his own armor. This means that what they wore was by no means standardized. Surviving illustrations and text, for example those of Paul Dolnstein who served as a Landsknecht on several campaigns and made both illustrations and writings of his experience, suggest that the front and sides in particular tended to be manned by more experienced soldiers with heavier armor whereas newer member of the formation who had less experience and armor were kept in the rear/center of the formation.
(Sidenote: If you do decide to check out Dolnsteins writings and illustrations, take his figures with a grain of salt. He gives 15000 for the Swedes at Älvsborg in the Danish-Swedish war for example, a more likely figure is some 3-4000. His illustrations are very interesting though. Here is one of his illustrations from the Landshut war, as we see here, these are clearly no unarmored Landsknechte. His illustrations also match up very well to surviving examples of Landsknecht armor.)
The Armory in Vienna has several fantastic examples on display ranging from early 15th century to late 16th century. What I found very fascinating in their display is that many of the armors show signs of modification throughout their use where the owner likely was able to afford some improvements to his armor. This is more common on armor owned by the individual soldier (such as the case of the mercenary Landsknechte) but less common on other types of munitions armor purchased by lords or royal armories.
