I work on the academic side of music, and much of my engraving work is creating musical examples for papers, handouts, exams, and so on. Sometimes, I just really, really want to hide an element without any fuss–perhaps I’m used to it from (ahem) other programs, but it was one of the features that I used most often. Some scenarios where this is a handy ability:
Creating an incomplete progression (e.g., soprano and bass only) with a hidden answer key.
Replicating an older edition exactly.
Including a single measure of a different key or time signature.
Hiding cautionary key and time signatures.
I realize that there are ways to accomplish some of these tasks that preserve the integrity of the compositional process, but, when I’m making things like musical examples, I don’t use that process. And I love the engraving quality of the program! It’s just a matter of flexibility.
(I did, by the way, search the forum for this topic and found a few things, but not exactly what I was looking for.)
I’m sure in the fullness of time all of these things will be possible, but we don’t want to take the approach of drawing things in grey when they’re hidden, with all of the attendant problems that causes in spacing, etc. You can see the beginnings of the approach to how we’re going to make this work with the signposts you can see for tuplets with hidden numbers/brackets, hidden time signatures, hidden accidentals, and so on. It will take time to continue to fit this to all possible items.
It’s difficult, I’m afraid. If you select it, you can see it. You might try filter-selecting all slurs in a range. But there’s no way to select only those with an alpha channel of 0.
If you use the hidden legato playing technique as @benwiggy suggests, you can use the view>signposts>playing techniques to show/hide them (limitation, this only shows the start, not the end)
Slurs and the Legato playing technique should be functionally the same. Check in the Key Editor that Legato is being applied, and that you have a Legato expression in your Expression Map.