Hide DONE todos of subtrees by setting a special keyword or tag - to support focus

Question

I would like to be able to set a keyword or tag for a subtree, which hides all todos in DONE states.

The Video of John Cleese about creativity states inspired me. He says that we enter a "closed mode" for getting work done, like going through a tunnel and at the end switch back in "open mode" for creative/planning tasks.

I would like such a "tunnel"-keyword or tag to help me focus on getting all tasks of a subtree done by hiding the DONE tasks (on the fly) as long as the keyword or tag persists. Maybe something similar to the COMMENT keyword before the heading.

I fear that archiving destroys the tree structure (e. g. for clock reports or later review). I would like to archive the whole project tree after a project is finished. Sparse trees don't update on the fly and I can not close and expand the parent heading without showing all the children again.

Example: 

open mode / normal

* Heading 1
** TODO One Thing
** DONE Other Thing
* Heading 2
** DONE Another Thing

variant closed mode - with keyword like COMMENT

* TUNNEL Heading 1
** TODO One Thing
* Heading 2
** DONE Another Thing

variant closed mode - with tag

* Heading 1 :tunnel:
** TODO One Thing
* Heading 2
** DONE Another Thing

I am new to Emacs.

Answer 1

I have a temporary workaround, which does not solve the problem, but lowers the impact of the cause.

I apply the ARCHIVE tag when marking a task DONE. Headlines with the ARCHIVE tag can be formatted specially.

(setq org-todo-state-tags-triggers
  (quote 
  (("DONE" ("ARCHIVE" . t)) ;; Set ARCHIVE tag when state changes to DONE
   ("" ("ARCHIVE"))         ;; Unset ARCHIVE tag when state changes to "" (no state)
  )))

Then I set the org-archived face to a light color via customize. I use a white background and also lower the font size a bit.

'(org-archived ((t (:foreground honewydew2 :height 0.8))))

Now DONE tasks are not really hidden, but loose a lot of visual attention. All headings with the ARCHIVE tag are now a lot less visible. Switching to the stateless state removes the ARCHIVE tag and makes the headings visible again.

For review I use Ctrl-c @ to mark a subtree. When marked the DONE tasks have a black color and are clearly readable again.

Headings with the ARCHIVE tag can only be expanded by pressing Ctrl TAB and not by pressing just TAB as usual. Subtrees of those headings are displayed with the normal face.

Update 16. June 2020: This method has some drawbacks since every task in every file is automatically archived and does not show up in agenda clock report any more and habits can not be tracked like this.