org-complete-tags-always-offer-all-agenda-tags

org-complete-tags-always-offer-all-agenda-tags is a variable defined in `org.el'.
Its value is

nil

Documentation:
If non-nil, always offer completion for all tags of all agenda files.
Instead of customizing this variable directly, you might want to
set it locally for capture buffers, because there no list of
tags in that file can be created dynamically (there are none).

(add-hook 'org-capture-mode-hook
(lambda ()
(set (make-local-variable
'org-complete-tags-always-offer-all-agenda-tags)
t)))

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.