Good question, I would think it does in some form or other. In fact Article 127/128 of the /Portuguese Penal Code give effect to amnesty within the criminal procedure.
Well done for finding that.
The massive publicity campaign has taken place, a large reward was offered in the past (whether that was a good idea or not). Amnesty for information might be a step, but I would have thought that it would depend on the circumstances of the individual. There might be a difference between someone not coming forward due to working without a valid work permit and someone wanted for rape or violent burglaries, for example.
An interesting article on cold cases says that people coming forward with new information is more successful in solving them than DNA is. I don't know how whether that is always true or whether it simply was in the context of the scope of that particular study.
http://news.nationalgeographic.com/news/2014/03/140306-cold-cases-murder-csi-forensic-science/Jim Gamble said a long time ago that people do change allegiances over time, which makes sense. A family member dies, a couple splits up, a former partner in crime or violent spouse is safely in jail...
On the other hand, omertà might be strong in small communities.
DNA might also provide a breakthrough, e.g., if some of these hairs lead to a suspect, that might also help take things forward.