Is the 60/40 screenshot rule an Apple requirement?

What is the actual rule?

Guideline 2.3.3, in full:

Screenshots should show the app in use, and not merely the title art, login page, or splash screen. They may also include text and image overlays (e.g. to demonstrate input mechanisms, such as an animated touch point or Apple Pencil) and show extended functionality on device, such as Touch Bar.

Two things are worth noticing. Apple writes should, not must, and the second sentence explicitly permits text and image overlays. The guideline that supposedly caps your marketing copy is the same one that grants permission for it.

Where did 60/40 come from?

From ASO practitioners, as a rule of thumb that makes 2.3.3 easy to satisfy without thinking about it. Keep most of the frame on real UI and you will never be near the line. That is genuinely good advice. It is just advice, not a specification, and treating it as a hard threshold leads people to redesign working screenshots to chase a number nobody checks.

What actually gets rejected?

Apple names three examples in 2.3.3 itself, and they are all about a frame that shows something other than the app working:

• Title art in place of the app.
• A login page.
• A splash screen.

The related failure comes from 2.3 itself, the Accurate Metadata section that 2.3.3 sits under. It names screenshots directly and asks that they "accurately reflect the app's core experience", which is what rules out mocked-up features the app does not actually have.

So how should you use the heuristic?

As a default, not a constraint. Lead frames often carry more marketing weight and that is fine when the app is still visibly present. If you find yourself trimming a caption to hit a ratio, you are optimizing for a rule that was never written down.