+1 for Python 3.7. Now would definitely be a good time.

There are probably a lot of addon devs who would love to use the scene update handler but don't understand how it works, or they are using it incorrectly. The idealized function of the scene update handlers should still be included in 2.8, just in a less hacky way. The intended function of the scene update handlers should also be included. Specifically, when the scene is updated, the handler should run.

This issue is part of a deeper problem. Sockets are not uniquely named. Unlike everything in else in Blender, multiple nodes can have the same socket names (even the same node can have the same socket names). I don't think this is good or consistent behavior. There's three things that could solve this issue...

scene_update_pre & post, while used as hacks, actually provide a useful pipe into blender's update logic that would make some specific scripting tasks impossible. Clever usage allows operators to be run out of context, let's a dev run asynchronous requests and smart threading, as well as code injections. Until there is a proper proposal for replacement handlers, scene_update_pre/post could be left in 2.8, as at least to my knowledge, there are several addons and plugins in production environments that take advantage of them.

Operator requires an execution override to work properly:

When called from the Dynamic Context Menu (spacebar), it works just fine

Do you have any settings enabled in the Color Management tab? If you are using Film, RRT, Log, or Raw color management settings, or have color curves enabled, or you are using one of the built in camera filters, Blender will crash upon rendering your scene. (These settings can be found in Scene Properties, next to the World Properties button, underneath the Color Management tab)