I may be being pedantic, but after reading that article it looks like there should be a fourth category: providers who do no accept Medicare under any circumstances. I see two types of opt-in and one opt-out, but no "we don't accept it, period." And the weasel words "formally opted out". What about informally opting out by just not taking on Medicare patients at all?

Edit: this article says that 1/3 of all primary care doctors will not accept any new Medicare patients. https://www.hlc.org/news/more-physicians-no-longer-seeing-medicare-patients/