They had existed in force since the fall of the Qing Dynasty if not sooner but someway somehow, they all seem to disappear after 1949. What happened to them? Did they turn communist? Flee with the KMT? Were they eradicated by the PLA?
For the most part, the warlords had been subjugated nearly two decades prior to the victory of the Communists in 1949, and did not exist as independent forces by that point.
The height of the "Warlord Era" was in the 1920s, and the major forces were either defeated by the KMT, or else had accepted KMT leadership and had their forces folded into the National Revolutionary Army by 1930. There were some uprisings following that point, so you certainly can't call the KMT a stable organization, but after 1930, the Communists were the main focus, and then after the invasion, the Japanese. In theory, what minor warlords still existed were at least supposed to be subservient to the KMT leadership (although internal conflicts of leadership were a major issue through 1949), so either found themselves on the losing side in 1949, or were smart enough to bring about a switching of allegiances.
While it more concerns itself with the organization than the campaigns, Chinese Warlord Armies 1911-30 by Philip Jowett is an OK introduction.