There is another odd thing with resources that was either changed for BtS or is very counterintuitive: One cannot trade obsolete resources and ALL trades get canceled automatically (which is very annoying) when one researches e.g. plastic and has a "package deal" with an AI involving fur. This does not make sense. The obsolescence should only mean that one does not receive happy faces from access to e.g. Ivory or Fur anymore but an AI without the respective tech would still get them, so why not trade them? (Or does the knowledge of the tech by *any* Civ make alle furs on the map obsolete?
It is similar with wonders. They only get obsolete if the *owner* gets the respective tech. This is often also counterintuitive. E.g. having Rifling does not remove the defense bonus from the AI who has Chichen Itza when I want to attack them with e.g. Riflemen... It would also be cooler if one could stop the use of the wonder for an AI by beelining certain techs. (I think in at least one earlier Civ game it worked like that: If any Civ got the respective tech the wonder became obsolete).
(The other annoying thing is that I always tend to forget how this works. (Got Rifles, screw your defense bonus) That's why I think that it was different in an earlier game or in vanilla.)