Whitepaper §7 reclaiming disk space. Once the latest transaction in a coin is buried under enough blocks, the spent transactions before it can be discarded to save disk space. If this was implemented in Bitcoin code from the beginning, the block size deba

However, the implementation was not ready on early days, and Satoshi didn't give an explanation on how much transactions is enough before the blockchain would discard earlier transactions and save only root hash, to keep the chain of Digital signatures. My guess is that the number of blocks that were initially configured for a difficulty adjustment moving average is a good pick to also use to discard earlier transactions.

The mining difficulty of Bitcoin was adjusted around every two weeks (or more precisely, every 2016 blocks) to maintain the normal 10-minute block time.

Bitcoin Verde node has released recently a version that supports reclaiming disk space described in whitepaper. I don't know what method it uses to find out if transactions are buried under enough blocks, but sure it is a good example of how Bitcoin should work and scale.