We have been told many times by numerous RCs that in order to be stricken from the RCC membership roles a person must write to their bishop and formally withdraw from the faith in order not to be counted as a member.
How many people go through this process? I know dozens of former RCs who are now Protestant and not one of them did this.
It was enough for them to shake the dust from their feet and read their Bible which led them to a Christian church where the Gospel of Jesus Christ is preached in truth.
What you're talking about is a formal act of defection. Yeah, write the bishop and a notation will be made in the baptismal register of the parish of record. That has nothing to do with the active register by which census numbers are reported. Only one parish can count any individual at a given time, and only one parish would want to.
Each parish keeps sacramental records and an active parishioner roll, annually the parish will report baptisms, marriages, receptions into full communion, funerals, etc as well as "active parishioners". The "active parishioner" number is where the number of practicing Catholics comes from. If they just totaled up everything there'd be more Catholics on earth than people.
St. Whoozits doesn't want you on the register if you're going to St. Thingys because that'll mess up the parish census for St. Whoozits and because St. Whoozits might get stuck with the subsidy for you if you enroll your kids in St. Bobbys parish school. Nevermind that the pastor of St. Whoozits will have to face an angry bishop if he's got 10,000 people registered as active parishioners and his weekly collections are only coming from 500 envelopes.