I think the foiled tag is helpful, because it says while the content will be heavy and uncomfortable at the very least the act was stopped. That's slightly easier to mentally prepare for than the tag that indicates that the act happened.
For similar reason I think a suicide attempt tag would be helpful. At least going into a story knowing that the act didn't succeed, it would at least be a little easier to start a story revolving around suicide if I know that they don't die.
I think that information is helpful for topics that are uncomfortable.
Now I can't speak with much authority on the topic, as I do not suffer from PTSD (as far as I know), but it's not about "how uncomfortable will this be". It's a content warning for people who, for what is essentially medical reasons, can not safely subject themselves to such content, which to my understanding does not necessarily hinge on the outcome of events in the story.
For everyone else, preparing for the worst and then being relieved that things didn't turn out to be as bad as feared should not be a problem.
As a counter point, a rape or foiled rape is ore likely to happen towards the start or middle of a story, while a suicide or suicide attempt will either be the inciting incident or climax. That means splitting rape and foiled rape attempt spoils less than a similar split for suicide would. I'd also argue there's a bigger difference in impact between a rape and a foiled rape, compared to a suicide and foiled suicide, to the point where a foiled rape can even be played for comedy in a way that none of the other three can.
I really don't think that's all that important. Besides, we have plenty of stories involving suicidal ideation, attempts and also concluded suicides (usually of side characters) early on, or even as a kickoff for the story, rather than at the end.