The last block that did not have BIP91 rules active was the one with height 477119, with hash 00000000000000000022552c92fdc5ac6c31a95f54d9ed9fcdf0fe00ff134773.

BIP91 uses BIP9, but with modified period sizes and threshold. It uses windows of 336 blocks, and requires 269 blocks to signal on bit 4. The window that ranged from block 476448 to 476783 (inclusive) had 284 blocks that signalled on bit 4. As a result, BIP91 locked in, and the next window of 336 blocks, ranging from 476784 to 477119 (inclusive) was spent in the locked in state. As of block 477120, BIP91 transitioned to the active state, and nodes that implement BIP91 will require signalling for bit 1 as of that block.