In AppiumLibrary, as of version 1.4.5, a very handy keyword called Element Should Be Visible has been introduced. But I am looking for the opposite version of that keyword too, which would be something like Element Should Not Be Visible. Since AppiumLibrary doesn't have that yet, is there any way we could achieve this?
There's always the possibility of expanding the library's python code, this shouldn't be very difficult. You basically just need to clone the Element Should Be Visible keyword definition and change the condition to do the opposite.
But, if that's not a possibility for you, perhaps you could use the keyword Run Keyword And Expect Error, in combination with the keyword you mentioned, Element Should Be Visible. Using this keyword on an element that isn't visible would throw an error, which in this particular case would be the desired outcome.
Still, that's a fishy workaround which will not help the readability of your test, and you should consider first looking into expanding the library itself.