@twMat The full concept belongs to buggyj. He shared the javascript code and CSS in this post for the collapsible outliner.
I just created a viewtoolbar button to alternate between normal tiddler body viewtemplate and custom view template cascade for collapsible outliner.
Since I was not the original creator, I just made a demo of the collapsible outliner in one of my tiddlyhost wikis for the participants of that particular thread to see. I used the filter pill macro from this post for easy drag and drop of this functionality (at that time I was organising tiddlers more using fieldvalues rather than tags).