Populate a table in Koha database listing both system preferences and to which version they were first introduced and perhaps also when decommissioned. This is valuable when planning upgrade to understand which system preferences to pay special attention to.
I think maybe not something for the database - documentation should better be kept outside. A pref can also be backported in some cases, which makes it harder to tell when it was first introduced. I think a script generating something from git history similar to what is used for the release notes might be a better way.