As Pseudonymous already stated, pruned nodes do not advertise the
NODE_NETWORK service, which indicates that a node will serve the full history of blocks. Corollary, a pruned node will not assist other nodes in their initial synchronization.
However, since Bitcoin Core 0.16, pruned nodes advertise
NODE_NETWORK_LIMITED as specified by BIP159. A node advertising
NODE_NETWORK_LIMITED indicates that it will serve the most recent 288 blocks. The pruned node can thusly serve peers blocks to catch up on gaps of less than two days and service requests for Merkle branches by light clients covering that period. Pruned nodes also relay information about peer nodes, transaction inventory and new blocks.
Even if pruned nodes have more than 288 blocks worth of data, they will not serve larger depths to prevent fingerprinting per the amount of data they store.