I agree with the need to easily hide things for educational purposes (I’m a university professor in composition). One case for example is figured bass harmony exercises. In Sibelius, I would write out the bass voice, then harmonize the whole thing in 4 parts to create an answer sheet. Then I can filter and hide the 3 upper voices to print out the assignment sheet, and unhide them to print the answer sheet - it’s all in the same file.
In Dorico so far I created an assignment file, then save as as an “answer” file and complete that one, but then I need to keep both in sync if I make a change.
I also have the case where part of the figured bass would be given, but part would be free to harmonize by the student - ideally I would put notes/rests to have the right music spacing, but hide them to leave it open for the homework.
In composition classes, I’m also teaching correct notation, and one could even think of a case where I would like to hide some required elements and have the students correct the example by filling in missing notation elements.
So there are cases where we would want on purpose something that was not perfect notation destined for performers.