>>>> - If completions-group is also non-nil, then group candidates according
>>>>   to their prefix and trim the prefix in the group-function when
>>>>   transforming candidates for display.
>>>
>>> The disadvantage of completions-group is its too wide coverage.
>>> We should strive for more specific options where possible.
>>
>> Right.  The advantage, on the other hand, is that you can toggle
>> completions-group on the fly in the minibuffer, either with
>> toggle-option or with a dedicated command.  The wide coverage of
>> completions-group means there's just one variable to toggle, always.
>
> Interesting.  Then this requires such precedence (from high to low):
>
> buffer-local value of completions-group -> metadata -> default-value
> of completions-group

The patch below doesn't yet implement this precedence since
I can't find a dedicated command that toggles completions-group
on the fly.  Such a command doesn't exist even in your branch
'feature/minibuffer-completion-enhancements'.  And anyway
this should be improved at the same time with introducing
a support for toggling the sorting order, layout, etc.

>>> Or better: let's enable groups by category.  I don't know why we have
>>> such a glaring omission that groups still can't be enabled by
>>> category.  This should be simple to implement:
>>
>> Doesn't the group-function metadata entry give enough control already?
>
> It's not easy for users to customize group-function metadata
> by writing own group function.  Whereas with a boolean its easy
> to toggle it in the Customization UI for completion-category-overrides.

Regarding the value 'group' of imenu-flatten, it can't be removed
since it's used in imenu--flatten-index-alist, so the patch
hard-codes ':group-p t' for the value 'group'.

>> If a category or a specific completion table wants to disable grouping,
>> it can simply avoid providing a group-function.  If it wants to enable
>> grouping, then it does provide a group-function, and now it's up to the
>> user's preferences which they express by setting completions-group.
>> Maybe your suggestion yields more flexibility in some cases?
>
> It makes sense to provide a group-function disabled by default
> to help users just to enable it in completion-category-overrides
> instead of writing it from scratch.

Here is the patch that supports

 (setopt completion-category-overrides '((unicode-name (group-p . t))))

without enabling the global 'completions-group'.