Block size limit in Bitcoin v0.01
What is the significance of this last line?


It ensures that the built-in miner does not create blocks that are larger than MAX_BLOCK_SIZE. It checks whether the transaction selected be larger than the limit. The reason there is a - 10000 is to ensure that there is enough space for the coinbase transaction and the block header.


  How does this impact consensus?


It's effectively a soft fork. Miners won't make blocks larger than MAX_BLOCK_SIZE. Otherwise there is no effect on consensus.


  Where is the code limiting block size in the most current version of Bitcoin Core?


It's in CheckBlock(), the same function the size check was since a size check has existed. Of course the location of this function has moved. Currently it is here: https://github.com/bitcoin/bitcoin/blob/master/src/validation.cpp#L3093