[Laforet]

Before computerization of the finance industry, banks used to keep stashes of "ransom money" with all serial numbers already recorded so they could be tracked to some extent. This is no longer necessary as modern money handling equipment makes it trivial to keep a detailed log of every single bill they receive or give out.

And what if you forgo banks entirely? Well, governments have also been legislating to slowly stigmatize cash use. Google "civil forfeiture" and you will find countless examples of the police taking people's money away without due process, as if carrying large amounts of cash itself is a crime.