Embedded Steampunk (Developer Extensibility)
What it is
Developer extensibility inside S/4HANA itself: writing ABAP Cloud code on-stack, in the same system as the standard core, using only released APIs. 'Embedded Steampunk' is the informal name for this on-stack ABAP Cloud.
Why it matters for Clean Core
It lets you keep performance-sensitive, tightly-coupled extensions on-stack while still being Clean Core — you get the cleanliness of ABAP Cloud without the latency of going side-by-side.
Common pitfall
Assuming on-stack means you can fall back to classic ABAP 'just this once'. Developer extensibility is still ABAP Cloud: released APIs only, Cloud Development language version. The location changed; the rules didn't.