Around here, contrails persist longer when there are
cirrus clouds. This means atmospheric conditions at the cruising altitude for commercial aircraft are suitable for forming clouds made of ice crystals. On warm clear days, the contrails dissipate rapidly. Jets have been producing contrails for as long as there have been jets. One of the combustion products of jet fuel is water vapor, and that is what becomes the contrail (condensation trail). I find amature analysis of pictures of contrails unconvincing.
The question is whether there is anything extra either in the jet fuel or dispersed via a separate mechanism. If there is a separate mechanism, somehow a tank and pipes must be added to an airplane, the tank must be filled, and the pumps and valves actuated in flight to begin spraying. It is unlikely that a pilot would be unaware of this because significant weight changes would be involved. The process of filling the tanks would be visible at the airport and would require a separate process from filling the fuel tanks.
If something is being added to jet fuel, then somewhere in the delivery chain the material is being mixed into the fuel. This would be difficult, but not impossible to keep secret. Any such material would need to not be damaging to the jet and not affected by high combustion temperatures (this tends to rule out this approach for distributing biological materials). Any airport workers out there who could get a jet fuel sample?