In 1.13 (or release 18w03b) Observers can be used to detect Pumpkin Stems and Melon Stems updating state to when they have bent and created a pumpkin. Currently, the Server does not use this update which means that people are using redstone for farms instead of observers, and therefore in a bid to reduce the lag on the server, this is a change that could be implemented.
An example of a design that can be used in this release is shown:

Thank you,
Stan
(Craney100)

 

No support, plugin reasons and more, it also won't change much.

 

Neutral I feel like updating this would bring more lag due to the server being optimized for 1.12.2

 

You can use the observer on the spot where the pumpkin would spawn. (They only spawn on dirt)