Ouch, that's weird! Junk jacks? I've used deoxit, CRC and Radio Shack contact cleaner, never seen anything like that.
Contact cleaner is generally safe to spray on electronics, it's mostly alcohol. But maybe polyester or other ancient "plastics" could be affected by it. Maybe it's something like gelatin or bakelite. Penny saved, penny earned, right? There are mfgs that will skimp like that.
But all quality RCA jacks, even the inexpensive ones like Switchcraft, use a modern dielectric that can take the cleaner without damage.
If the plastic dielectric is that bad, I would be concerned about the metal used in the contacts, and the wire used in the hookup, etc. It might be steel instead of brass. PE sells Chinese made speaker posts made out of cast zinc. The gold paint flakes off, and it snaps if you tighten it too hard and compared to real metal they sound awful. There's a lot of junk out there now.
WD40 leaves a lubricant behind. Contact cleaner leaves no residue.