>>   (defcustom read-char-by-name-group nil
>>     "How to group characters for `read-char-by-name' completion.
>>   When t, split characters to sections of Unicode blocks
>>   sorted alphabetically."
>>   =====================
>>
>> It seems a new function is needed to sort groups, e.g. `group-sort-function`.
>
> I think whether the groups should be sorted alphabetically or "by order
> of appearance" can be a choice made by the UI rather than by the
> completion-table.

Then this patch adds a new option read-char-by-name-group-sort,
instead of read-char-by-name-group now replaced by completions-group: