Author here. I think it's a pretty nice idea to include deprecation information in the response headers as another way to keep API integrators informed - alongside written forms of communication like blog posts, changelog entries and emails. We'll discuss that internally.

As @xavdid noted, the reality is that most people won't see these headers, but it's a nice power user feature.

I like the idea of this, but in practice, I'm not looking at responses for an API unless I'm actively developing on it. If I'm doing a maintain-occasionally project, then I'll never see the deprecation response. I'm also unlikely to set up an automated alert for a random API I used one time (tricky to do without external tooling, like Zapier). I'm much more likely to plug an email into a form, since that's all set up for me already.

That said, as long as there's multiple ways to get notified, more is probably better.

This is a neat idea. Are there examples of this in the wild?

Author here. We actually use deprecation headers in our GraphQL API.