TL;DR;: Workflow Multibranch job can't be disabled from UI however call via script console works as expected.

Workflow Multibranch plugin view is derived from ComputedFolder implemented by Folder plugin which supports this behavior (ComputedFolder contrary to AbstractFolder uses as a default ON) . Everything is already prepared/implemented, the issue is only the fact, Jelly can't call a method with protected scope - exception is probably occurred, so any test utilizes a call supportMakeDisabled() for this is recognized as 'false'/'otherwise' and the result is it doesn't work as expected.

I've heart some rumor there is an objection against this feature, but I can't find any relevant source.

What I'm trying to solve is the real customer use-case - the Jenkins deployment is orchestrated by JCasC but for develop/ci/debug purposes we want to disable all jobs loaded by a SEED job due to resources utilization etc. We don't want to automatically execute any job (triggered by any internal/external trigger or event) in development environment. Any type of job like Matrix, FreeStyle, Workflow etc. has this feature/behavior and it is exposed to UI as expected. I don't see any reason to not have the same behavior for a Multibranch kind of job. I don't speak about disabling particular branch, I speak only about disabling the whole job itself.

I see this as easy fix for the plugin. If there is an undiscovered dependency/side-efect, lets talk about it here.