It’s been suggested to me that colleges sometimes pay the College Board and ACT to receive ALL of an applicant’s scores, so even if you use score choice they will see all of your scores. Is this true?
No, though they may be on your transcript from the HS.
College Board and ACT provide nothing to colleges to show you even registered for a test that you choose not to send and no college can pay to have it otherwise.
A college can find out your scores if you attend one of the many high schools in the nation that put all your scores on your official transcript sent to colleges, so you better check with your high school to determine what it puts in the transcript. In fact, there are still many colleges today that do not even require you to send scores from the testing agency if they appear on your transcript (e.g., one of the ivies, Brown, is included in that group),