Neutral pigtails on MWBC
Multiwire branch circuits (MWBCs) share a neutral.  There is a code rule that prohibits wiring devices, such as receptacles, in a way that the removing the device would open (i. e., disconnect) that shared neutral.  If the device is pigtailed, that rule is satisfied because removing the device doesn't disconnect anything else on the circuit from the neutral.
The prohibition exists for MWBCs because opening the neutral causes major trouble on those circuits.  When the neutral is opened on a MWBC, the voltage at the outlets destabilizes - on a 120V MWBC, you could see anything from 0V to 240V*, causing all kinds of problems.  So if someone removes a device and unwittingly caps the neutral, leaving it open, you can wind up with a very bad situation.
Obviously if you wind up with 240V applied to appliances and etc. intended for 120V, they may by destroyed immediately, especially sensitive electronics, and think how many things around the house have sensitive electronics.  Motors may run way too hard for a short while, then fry.
A device like a space heater, a purely resistive load, may not be destroyed or damaged, but will pull up to double the usual current, overloading the circuit.  Hopefully the breaker is functioning correctly and it will trip quickly before anything is damaged.  But if anything is wrong with that breaker, it could fail to trip and cut power, and the resulting overload can damage wiring and connections, and is a major fire risk.
I wouldn't let this go for long.  The risk is just that someone working on the wiring removes a receptacle and unintentionally leaves the neutral open.  So until it is corrected, I would not allow any other work be done on the wiring.
If everything is nice and neat and roomy like the box in your photo, it's likely a day's work with very little materials required.  Open the box, use the one pigtail that's already there, add a pigtail to the other receptacle, nut the four together, close the box, lather, repeat.
By the way - there is no 2018 NEC.
*With the neutral open, a shared neutral circuit degenerates into a 240V parallel - series circuit, such that the loads (in parallel) on L1 are collectively in series with the loads (also in parallel) on L2.  The voltage on the L1 side and the L2 side will depend on the load on each side.  If there are no loads on L2, the L1 side will see 240V.  If the loads on the L1 and L2 side are the same, the voltages will be the usual 120V on both the L1 and L2 side - until something is turned off.