Various Star Trek series and movies have shape-shifting species, but do the Changelings appear in canon outside of Star Trek: Deep Space Nine? If not, has there been any reason stated for this? Major alien species often appear again in later series, but I am not aware of the Changelings appearing after Deep Space Nine.
At the end of DS9, the Founders sign an instrument of surrender known as the 'Treaty of Bajor'. A closer view of the prop shows that one of the key stipulations is that all Founders (and their vassals) must leave the Alpha Quadrant immediately and remain in the Gamma Quadrant.
This would obviously explain the conspicuous absence of any Founders in future episodes of Voyager or the subsequent theatrical films.