The initialblockdownload field in the output of the getblockchaininfo RPC reports the internal “IsInitialBlockDownload” state of the node software.
The value of this state is always true at startup, and latches to false whenever both of these conditions are true:
- The currently active chain has at least as much cumulative PoW as the hardcoded constant in the software (updated every major release).
- The timestamp of the currently active tip is no more than 24 hours in the past.
Whenever the IsInitialBlockDownload state latches to false, it never reverts to true (unless restarted).
