https://github.com/Vazkii/AutoRegLib/blob/master/src/main/java/vazkii/arl/network/NetworkMessage.java#L57

Since message replies are broken at this point in time (ref MinecraftForge/MinecraftForge#4231), in my opinion the line above should call to IThreadListener.addScheduledTask to avoid child packets accidentally executing handler code on the network thread.

Is that still applicable?

There has been no change to the linked code, so I would assume yes.

All the handlers take care of scheduling anything necessary onto the ClientTicker, so this isn't an issue.