When it comes to actors, it really depends on the situation. There are a lot of examples of actors who leave a show, and then come back later in a one off or recurring capacity. On the Trek shows, Denise Crosby, Wil Wheaton and Jennifer Lien left, and then came back at least once.
I've been watching The Waltons on MeTV for the last few months, and that had both Ellen Corby, who played the grandmother, and Michael Learned, who played the mother, leave, but then still pop back up for a few episodes later.
In the original CSI, Jorja Fox left the show and then a couple years later she came back full time.
24 killed off Carlos Bernard as Tony Almeida in Season 5, but then he showed up alive in Season 7, and popped up a few more times later on in the franchise.
I think the fact that they are having Kate disappear, rather than killing her off, could mean that they are leaving open the possibility of Ruby Rose showing back up in some capacity later.