Do all cells release exosomes?

No, not all cells release exosomes. Only cells that have an endomembrane system release exosomes. This includes most mammalian cells. Some cells such as dendritic cells, B cells, and mast cells of the immune system are found to release exosomes constitutively as a way to communicate with other cells in the body. The extent to which non-immune cells may release exosomes remains largely unknown.