So people are wanting to go back and find their dead horse .. is the horse's saddlebag part of the settings_deadnpc_despawntime= timer in server.properties? Will this setting hold the inventory on the horse too?
The setting "settings_deadnpc_despawntime" only determines when the dead horse body despawns (in seconds). Once a mount dies, you can get the saddle (and saddle bags) back from his dead body. If there were items in the saddle bags, you find these items also in his dead body. But once the body despawns, there is unfortunately no chance to get the items back.
If you set the despawntime to -1, the body will never despawn (unless the server restarts). However, this affects all dead animals, so this might clutter the server over time. It's recommendable to use a value like 300 (5 minutes) or 600 (10 minutes) etc