I'm in a relationship and my partner knows that I want to die. It's not necessarily something you can keep a secret in a long term relationship, and I've had ideation long before I met them. Due to my ideation I wasn't planning on getting into a relationship but shit happens, lol. I don't really have much of an opinion on it. I know that it makes them sad to imagine a future with someone who doesn't imagine a future at all, but there's a catch to being with anyone and I always leave the door unlocked for them to leave if it becomes too much. But my partner has basically created an obligation for me to stay alive and I kind of regret that aspect of it. It's not something cute like a promise or anything lol. A literal obligation that has me tied for at least a decade more. It's very unfortunate. I can't even leave the relationship with a clear conscience, now.
We are all going to bring someone in our lives and surprise them with a death at some point. Death is the natural end to all of our lives. So I don't really worry too much about it. It might sound cynical but they're going to lose me in one way or another, and they're going to have to deal with grief in some capacity when it comes to me. Everything that I do is to prepare them for a stable life without me and sometimes I resent doing it because it feels against my will due to the situation they mostly created for us, but they don't know that and I will not worry them with that knowledge. They're understanding of how I feel as they also have been suicidal before and have their own mental health issues to deal with, so if there ever comes a time where I decide to put my plan into action, I hope they can understand why I chose to go out in my own way. And if it happens sooner rather than later it will probably be because we've broken up and I'm no longer tied to that obligation, anyway.
I do agree that if you're planning to ctb then you shouldn't bring a partner or a child into the equation. They are a real damper on the plans and some may consider you selfish for doing so. I'm a cautionary tale, but it isn't so black and white. Things happen and sometimes they are out of our control.