Inquiries about item loss of effectiveness

It's been in the equipment for a long time
The effect of Soul Core of Jiquani is changed to blank and no effect is being applied.

There is something in the Third-edict patch notes about it applying to chest armour only and not all armour. Maybe has something to do with it.

https://www.pathofexile.com/forum/view-thread/3826682

"Soul Core of Jiquani now grants 3% increased maximum Life on Body Armour (previously 2% on all Armour)."

You need to ask in the early access sections though, this section relates to POE1.

So there was no reward for the existing unusable soul cores?

Not sure. Try early access section.

https://www.pathofexile.com/forum/view-forum/2217

If you are saying an old version now does nothing, I'm not sure why it would be left in the game rather than removed. Maybe so people have time to understand there was a change. Perandus coins still exist I think, even though they don't have a current use. You should try and get an answer from POE2 section. I'm not across any changes that may have occurred or if it has a purpose. Older patch notes may have some information if it changed prior to third edict.