To my knowledge, there hasn’t been a major peer reviewed study to show whether these warnings make any difference.

Now, my own anecdotal non-peer reviewed personal opinion would be that they probably make no difference at all. Businesses likely began adding them only to waive potential liability and not to actually do anything helpful. They can be frustrating because they spoil upcoming events in media that may have been unexpected or unknown, but because of the warning are now definitely known and thus feels “ruined” when it happens. They can also reinforce ideation of suicide because a person may feel like the ones that added the warning did it as a token thing, treating the person like they are a badge of honor or some kind of selling point. Whether that is true or not doesn’t really matter, a person that is suicidal is almost never “in their right mind,” and if they feel that way, they feel that way. Nobody can tell them how to feel, not even themselves sometimes.