When was getheaders added? Bitcoin 0.10 uses getheaders to synchronize. How long has this message been supported by other clients? How many existing nodes support it?
How long has this message been supported by other clients?


It was added by Satoshi in Bitcoin v0.3.18 (released on december 8th, 2010)


  How many existing nodes support it?


All of them. It has been used by SPV clients for a long time to do fast synchronization without downloading all historical blocks.