The core class BacklinksCache currently has __sleep method with clear intention to support php serialization of its instances. However, looking at all the usages of the class, I do not see why would we want to serialize it. Serializing should only be needed if it's ever saved in a property of another serializable class, and I can't find a case like this.

• Find usages of BacklinksCache and see if it indeed needs to be serialized. If it is, scratch the next points and think more about it
• Create DeprecatedSerializationTrait - will emit deprecation warnings form sleep and wakeup. See NonserializableTrait for inspiration
• Use the trait on BacklinksCache, deploy, wait one week
• Use NonSerializableTrait on BacklinksCache.