I wonder if the next step for AI companies (those in control of search, at least) will be to start refusing to index content from companies that have opted out, in an effort to sway their decisions.
So basically just google? I don't think other search engines matter that much. I've never heard people specifically doing SEO for anything that's not google.
That would probably be very effective so I think there's a good chance Google does take that step at some point. The only reason they might not is that it looks extremely anti-competitive to leverage their search monopoly in that way, but we all know how lax enforcement is in that area these days so I would be surprised if that stopped them.
So basically just google? I don't think other search engines matter that much. I've never heard people specifically doing SEO for anything that's not google.