Looks like it was introduced back in 2017 along with Apple Pay
https://www.apple.com/newsroom/2017/12/apple-pay-cash-and-person-to-person-payments-now-available/
Discussion
Looks like it’s US only. I’m in the UK maybe that’s why I don’t have the option for it or even see it on my phone. Not that I would dream of using it anyway.