The last ice age significantly impacted human migration, primarily by creating land bridges where oceans had once been. For example, during the last ice age, lower sea levels exposed land bridges like Beringia, which connected Asia and North America. This allowed human populations to migrate from one continent to another.
While the other options you've mentioned—creating new shelters, forming larger communities, and living closer together—may also have been influenced by the environmental changes during the ice age, the most direct effect on human migration itself was indeed the creation of land connections that facilitated movement across previously inaccessible areas.