Other AMEX cards had this offer before the end of the year. Blue Cash just started it. Plan It is the way to break up larger purchases over a number of months - 12 is usually the lowest offer but I think it can go over 36. For Blue Cash through the end of April, you can set up a Plan for no fee. The payments depend on the months chosen but basically, it ends up being no fee, no interest equal payments over the time period.
The other piece is new purchases paid in the current statement period accrue no interest either. You will also accrue whatever the rewards as the payments are made.
There is no catch except that it means you’ll be keeping the card for whatever the time period or paying off the whole thing to close it. Also, if you are concerned about your credit score, it could lower your score if you are using much of your credit limit.
As always, please ignore this post if this is not useful for you.