When you are working on an upgrade, wouldn't it be good to "phase out" custom developments where possible ? OF COURSE IT WOULD ! SAP lays down the base system structure for your company to work with. And no custom developments are needed at all - to make this base system work. With some exceptions to this rule, custom developments are there for customer specific needs and these developments need to be upgraded years after they were build.
Whatever developments have been done on your system, an upgrade involves revisiting it all. It also gives you a chance to filter out the developments that are no longer used or no longer needed (because of SAP improvements). Have you ever found a tin can of food in your kitchen cupboard, which has been over it's consumption date for more than a year ? I bet many developments in your system have passed their consumption dates as well. The great thing about food in a tin can is that a consume-before date can be calculated. You will never know when a development is used for the last time. When you regard an upgrade as a house-move, you may be carrying around boxes of out-of-date developments and making sure they still work on your new system.
Back to SAP
Remember: if it is possible to phase-out a custom development it means the development will no longer require upgrade time (and upgrades are a recurring theme... ). Any decision you need to take on developments: choose the path that brings you closer to standard SAP !!
Grave yard
Just knowing which developments are in use is valuable information for the upgrade. A mechanism of phasing-out old developments needs to be in place on any serious SAP implementation. Check out the article Development graveyard on this