Thanks for your reply, especially the part regarding sending messages to contacts; I hadn't appreciated on a deep level how bad that could be. Prior to writing this post, I hadn't realized  how varied people's perspectives are on the topic of suicide prevention, and your comment (and others) made me realize that, if I choose to keep looking into this, I need to devote more thought and research toward the big picture stuff and talk to / read things by people with more direct experience before (potentially) speculating on possible interventions.

This is a good point. The Belfort et al. paper mentioned above implies that, among adolescents admitted to a certain psychiatric emergency room due to suicide concerns in 2012, at least 1% presented to the emergency room because, after communicating with a peer electronically, that peer "shared information with an adult or encouraged access to care," which suggests there's a fair bit of informal suicide prevention being done online that could potentially be disrupted by the knowledge of an automated service (though I guess texts also fall under "electronic means").

Thanks for your comment. I feel very grateful to have received such a thorough reply (especially from someone with so much experience in the area).

To be honest, I haven't looked carefully at most of the papers I mentioned here concerning intervention effectiveness, including Pil et al. As I mentioned in the post, I still plan to do a more extensive literature review. It's interesting to hear your perception on how academic experts feel about intervention effectiveness; I tried a bit to find a recent thorough review article on this, but didn't have much luck.

Regarding the question of whether suicide prevention is net-positive in the first place, as I mentioned in another reply below, I felt pretty convinced of this after casually reading this blog post (whose main argument is that most suicides are the result of impulsive decisions or treatable conditions / temporary circumstances), but I think it would definitely be worthwhile to go through the argument more critically.

I hadn't considered liability risks, and, though I guess what I was describing is more like a bot than an app, it's possible they would still be relevant, so thanks for drawing my attention to that.

Thanks for bringing this up! I found this blog post by Scott Alexander pretty convincing as an argument that the vast majority of suicides are the result of temporary or treatable circumstances / psychological issues, but I haven't gone through the argument very critically.