The federal law for cash reporting is any transaction in cash (or cash equivalents) that is over $10,000 is required to be reported.
I.E. $10,000 does not need to be reported, but once you get one cent over 10k with a cash transaction the Feds require a 8300 cash reporting form.
Everyone at my office (myself included) has to take an anti money laundering exam every year.
What the Feds consider cash equivalents are things like money orders where you buy a negotiable instrument with cash and the funds are not directed from a bank account.
Things like a personal check or bank wire the Feds do not consider a cash equivalent as a funds are coming from an account and presumably taxes were paid on those funds.

In regards to one of the earlier questions about breaking a larger payment (over 10K) into smaller payments that are under 10k.
This is called structuring and illegal.
It is however ok to make multiple payments to a person that total for over 10k, but they need to be unrelated sales.

I don't really use any of the "cash apps" other than sending the occasional paypal so I haven't followed what they are talking about doing in regards to reporting.
According to the IRS website the 1099-K is for cumulative transactions that total more than $20,000 or more than 200 transactions in a taxable year.
This sounds to me more like the IRS trying to crackdown on people runnings smaller businesses that are trying to fly under the radar.

At one point Bureau of Engraving and Printing did issue $500, $1,000, $5,000 and $10,000 notes.
They were last printed in 1945, but continued to be issued until 1969.
Their official reason for discontinuing them was "lack of use", which is probably partially true.
These bills were intended for larger transactions that are not "everyday purchases".
All of those denominations are still legal tender and can technically be spent, although the collector value is higher than the face value of the notes.
I've bought and sold many $500 and $1000 dollar bills over the years, as well as a few $5,000 and $10,000 notes.
When these bills were current most people had never seen a $5,000 or $10,000 and $500 and $1000 notes were not common use as this was a lot of money during that time period.
I did have one client once tell me that he recalled his father showing him a $10,000 note when he was a kid and sold their family farm and they went directly to the bank with it.
Unless you have a need to move around large amounts of cash, the larger denominations are not really that practical for everyday commerce.
I doubt we will ever see something like those denominations again unless inflation gets really out of control.

The BEP also printed $100,000 notes, but those were strictly used within the government and are not legal to own and they were never issued to the general public.

I think being aware of the laws is just as important as being aware of the scams that are out there.
Both are things that should be discussed and people should be aware of for a variety of reasons and their possible implications.