Any opinions on this?
I am having several issues, as part of developing complex plugins, by needing some strict configurations. And this issue is blocking the consistence of these.

IMO, transistions should comply strictly to the workflow settings:
a status withouth any outbound status configured, should not allow transitions.

@vboctor @dregad @atrol
Anny support for it?

This seems like a breaking change and may confuse users when all of a sudden they are not able to do things that they were able to do before. So it may be worth exploring other options.

One option is to enable plugins to filter the valid target statuses for an issue. This may be dependent on plugin logic, issue details beyond status, etc.

Se the attached screenshot for a bug in such status.
The change status control is not showed.
The status may be changed by group actions though, as that shows all status (regadless of workflow settings)

So one issue, is that the status selector should be available, according to current logic.

A secondary issue, but no less important:
There is no way to configure a status that has no valid transitions.

Selección_036.png (12,542 bytes)   

Selección_036.png (12,542 bytes)   

@vboctor
The empty option check was introduced here: 0014013
commit
241ae23b4912dbd83da3fa9e611322a1646e50b7

regarding defaults:
"By default, there is no workflow defined, which means that all states are accessible from any other, by anyone."

This is from config_defaults_inc
$g_status_enum_workflow = array().

Note this inconsistency again:
manage_config_workflow_page.php, for the default array(), shows all status checked. If saved, a local config is created with the filled matrix, even if no change is made.

I suggest setting this semantics:
1) IF status_enum_workflow == array() RETURN true
This means all transitions are valid, to keep this default working.

2) If any row is not set
Keep this check, for the case where new status are introduced, and still not updated in the config matrix

if ( !isset( $t_status_enum_workflow[$p_bug_status] ) ) {

return true;

}

• However, manage workflow page should show this case as a row with all status checked, same logic as (1)

3) make the workflow config save an empty array if no status are checked, so tha:
there exists a row $t_status_enum_workflow[$p_bug_status] (which is empty)
MantisEnum::hasValue( $t_allowed_states, $p_wanted_status ) returns false naturally

I have tested that hardcoding status_enum_workflow[STATUS] = ''
works in my expected way, where no transitions are allowed

Updated description with another inconsistency:

A status without a transitions row, which is defaulted to all transitions, it still shows:
"You cannot move an issue out of this status"
In manage_config_workflow_page.php

submitted PR: https://github.com/mantisbt/mantisbt/pull/836
with the changes.

It does not break current configurations and behaviour.