Update 12/9/19: Reader karma has received the same e-mail, but unlikely previous people they have not been skirting the rules on the referral cap. They received it on an account with a Chase Ink card where all the spend is manufactured spend at Office supply stores (paid off via bill pay at Walmart).
Update: I should have made this clearer, but this letter was sent out to people who were using methods to bypass the 5 card referral cap to earn additional points. Not just people who were hitting the 5 referral cap and others kept using their links.
Chase has sent out letters to some cardholders stating that they have been misusing the rewards program and if it continues all Chase credit card accounts would be closed. It seems that people are receiving this letter because they have gone over the maximum number of referrals you’re allowed to receive on a single card (5 in this case). There are a number of ways to go over this maximum, but I won’t go into detail here as you can see this is a bad idea and can lead to account shut down. The letter in question can be found here.
Hat tip to Andysol1983