Prior soft forks were activated via miner signaling in block version numbers. Now that the chain has long since passed the blocks at which those consensus rules have triggered, as a simplification and optimization, the trigger mechanism is replaced by caching the block heights at which those consensus rules became enforced. The change is merely cementing into place a few attributes of the blockchain's history that are not in dispute. There appears to be no disagreement amongst Bitcoin users that BIP34 activated at block 227931, BIP65 activated at block 388381, and BIP66 activated at block 363725. This change does nothing to provide justification for this consensus rule change, but it ensures that any reorg that would cause the activating block to be disconnected will raise fundamental concerns about the security assumptions of Bitcoin, a far bigger issue than any non-backwards compatible change.