Usually, carrying it extended will not directly damage the lens just from being extended. Several photographers report doing this for years without problems. The bigger concerns are practical ones: the extended barrel is more exposed, easier to bump into things, and can collect dust, grit, or sand on the exposed section. When the zoom moves again, that debris can be drawn into the lens.

So the answer is: generally no immediate harm, but it does increase exposure to dirt and knocks. Using the lens hood helps protect the front, and a hood cap or soft cover can reduce dust when walking. A holster or belt-supported carry method can also reduce strain and keep the lens more secure. If the lens extends under its own weight very easily, that is zoom creep rather than normal damage, but it can be inconvenient on heavy zooms.