How to switch a system off
Decommissioning is engineering work too. The systems that never die are the ones nobody planned to end.
Organisations are good at starting systems and bad at stopping them. The result is a landscape of tools that are "still used by someone", each costing licences, attention and integration effort.
Find the real dependencies
Switch off read access for a week before switching off the system. The complaints tell you who genuinely depends on it. Silence tells you more.
Archive, then delete
Export the data in an open format, document what it means, store it where legal and finance can reach it — and then actually turn the system off. A system in "read-only mode forever" is not decommissioned.
Celebrate it
Removing a system is a harder achievement than adding one, and rarer. Make it visible, so that the next team is encouraged to do the same.