Since when are headphones and such jacks lubricated
Since When??? Since ALWAYS.
The metal MUST be protected from Oxygen. The Oxygen corrodes the metal and stops the electrical contact.
The Oxygen breaks down the plating on the metal....that's why.
The lubricant seals and protects the surface of the metal, blocking oxygen.
Cleaner used for audio jacks and controls is ALWAYS lubricated.
NEVER apply non-lubricated "contact cleaner" to plastic parts. This can melt or damage the parts.
I hope that answers your questions.