I have few people who I trust and who have been accepting of my suicidal thoughts in the past. I'll let them know ahead of time, without any details, so they know what exactly happened if they lose contact with me. Though even if I don't, they'll figure it out.
Those whom I cannot trust to accept this, I'll let them figure on their own, if ever. It might be too disruptive for their lives to know, so I'll prefer to not push that information onto them.