DeathAndTaxes is probably the clearest writer on this topic.
It seems like to me that any implementation of block chain pruning, would in effect, create a decentralized checkpoint that could be leveraged to solve the "nothing at stake", past coins being used to attack the network issue in PoS. Why does NOBODY talk about this?
Perhaps I am not following. Pruning isn't at all like a checkpoint. Can you explain in more detail what you mean?
Satoshi describes how blockchain pruning works in Section 7 of the original white paper. Transactions are hashed into a Merkle tree, allowing for blockchain pruning without breaking the block's hash. It is actually quite mind-blowing how this works. Take a look:
https://bitcoin.org/bitcoin.pdf