I noticed a setting in the JSON config related to "decay." How exactly does it work? For example, if I set "decay" to true and "claimedLandDecayReduction" to 1.0, does that mean players would not need to maintain their base with resources in the tool cupboard?

Exactly. The "Building Decaying" would still show but decay damage would not be applied to structures inside claimed areas.

Note if any other plugin tries to also modify the decay damage, Imperium will do nothing and leave it to other plugins.

So in order for this to work you must be sure the only plugin messing with decay is Imperium