Actually, it’s not legal. Making repeated cash deposits/withdrawals just under the $10K limit is called “structuring” and is prohibited under the Bank Secrecy Act.
It’s only illegal if you are doing it specifically to avoid the reporting requirement.
https://www.offshorelivingletter.com/structuring-cash-deposits-illegal/
It is perfectly legal to make $9,900 deposits every day for a year or decade if that is just the money you have on hand.
Yes. It is legal. But it WILL get reported and I would behoove the person to have some good records on hand.
As I noted earlier, I am no fan of the reporting rules. But I will also tell you that being honest is easy. Knowing the rules is easy. I am not sure why everyone gets worked up about this in practice.
Most SS folks and all bankers will shake your hand if your business or personal dealings are generating that much cash.
And the drug dealers know the rules and don’t do this stuff. So...you will probably not hear about them getting caught. They have hundreds of accounts that some kid goes to every week depositing $1,100 and $1.756 ever week.