This Thread (already referenced before) makes a compelling case as to why Implosion is no death effect.
To summarize:

Death ward protects against "death spells" and "magical death effects".

- death spells are spells with the [death] descriptor, Implosion is not
- magical death effects are supernatural abilities that, while not spells, work like a [death] spell. They prevent raise dead. Any spell with these attributes is specifically noted as a "death effect" in its description.

Those are the basic "rule" arguments. But additional arguments make the case even more compelling:

- Implosion kills by physically destroying the body. All known [death] spells or spells that are labeled "death effect" belong to necromancy and directly attack the life force
- Death Ward would be OP if it protected against every single Save or Die spell
- If it did, it would also portect against spells like holy word or phantasmal killer, which clearly does not fit the fluff

The Thread also contains reference to an article by WoTC describing Implosion as one of the effects that cannot be countered by Death Ward.

All in all, the case for Death Ward not being designed to protect against Implosion is pretty strong. But that does not mean things are not different in the OOTS-Verse.