Is it true that McDonalds bread never gets rotten?
No, if there is moisture in the environment, mold will grow.
There are countless photographic and home-based experiments that have tried to evaluate the decay of McDonald's burgers. For example, the Happy Meal Project photographed a cheeseburger and fries for 137 days - with little or no noticeable difference.
A more scientific approach, where the variables are separated and tested individually, provides different results.
"More than three weeks later, the McDonald’s food hadn’t rotted, but neither had the homemade patties. The homemade patty with no added salt looked no different than the those with extra salt, indicating it wasn’t the causal factor.
The key appeared to be moisture levels. The burgers had each lost a quarter of their weight within the first week, indicating that they had dried out. Without moisture, the mold can’t grow. Since McDonald’s uses thin patties with a lot of surface area, they quickly dry out before they can start to rot. This is the entire principle behind beef jerky. A McDonald’s burger sealed in a plastic bag will be completely consumed with mold within a week." (IFLS link below)
Here the tested variables were salt content, moisture in environment, and the possibility of natural mold existing on the burgers at time of purchase.
The SeriousEats link below gives a great overview of a controlled experiment with home made and store bought burgers and buns.
They determine that "93% of the moisture loss in a regular burger occurs within the first three days". Without moisture, no mold can grow. The burgers are essentially 'jerky' in 'dried bread'.