Your S/4HANA custom code is frozen behaviour

I've sat in enough S/4HANA planning sessions to notice a pattern. We talk about custom code as if it's a technical problem. Mostly it isn't. It's frozen behaviour.

SAP ECC mainstream maintenance ends on 31 December 2027, about 14 months from now. Extended maintenance runs to end-2030 at a 2-percentage-point premium. DSAG's latest survey found only 37% of German-speaking ECC users plan to switch by end-2027, and almost half are aiming for 2030.

Buying time makes sense. Spending all of it on the technical backlog doesn't.

SAPinsider's 2026 research names custom code remediation as the biggest deployment hurdle. I'd put it differently. Every Z-report, bespoke approval step and workaround exists because someone once wouldn't, or couldn't, work the standard way. Sometimes for good reason, often just out of habit.

A clean core isn't an IT principle. It's a negotiation with hundreds of habits. Here's some that I've seen, and I'd hazard guess you have also seen in recent history:

The custom approval step that "Finance needs", which turned out to exist because Gary in 2009 liked to see every PO over €500 before lunch. Gary retired in 2017.
A bespoke report built to match the old Excel sheet exactly, down to the yellow highlighting, because "that's how the Monday meeting reads it". Nobody could say who reads it.
A €200k custom field for "Customer Mood", added so sales reps could keep typing notes where they always had. It's filled in 3% of the time, mostly with "ok".
Three different invoice layouts, one per country, that turned out to be one layout and two people who didn't want to change their printer settings.
A custom workflow that emails a PDF to a manager, who prints it, signs it, scans it and emails it back. The system already had digital approval. It was "a control".
The "critical integration" to a legacy system, kept alive for one user who still checked numbers there "just to be safe".
A whole custom transaction recreated so the screen looked like the 1998 green screen, because the warehouse team "knew where the buttons were".

The deadline is a date. Readiness is a behaviour.

So I'm curious: if you had to retire one "essential" customisation tomorrow, who would push back first?