There are plenty of living, breathing exceptions to that, myself being one of them. The bottom line is that it is at the discretion of the priest.
I agree with the RCIA process. I believe it is a very beautiful thing. My parish has a thriving and orthodox RCIA program. There are some cases where it is not necessary or practical or both.
In my case, after 3 years of regular attendance at various Orthodox churches, 8 months of catechesis in an Antiochian Orthodox church, with this whole time sprinkled with generous (name removed by moderator)ut from a very ecunemically minded Serbian Orthodox priest. When it became clear to me that God was calling me to become Catholic, I attended mass for several weeks.
I then shared this with my parish priest. I told him that if he felt it necessary, that I’d enter RCIA. He did not. And here I am.