Also: when the slider-untouched class is positions at the highest level, the designer can also ifluence the tooltip, the track, which could be used to better distinguish when slider questions have ben answered or not

I second this.
When adjusting CSS class names at such elements please keep an eye on backwards compatibility.

Using vanilla survey theme, but could not find .js related to untouched class of slider. Is this hidden deeper in the system?

The script is here : https://github.com/LimeSurvey/LimeSurvey/blob/master/assets/packages/questions/numeric-slider/scripts/numeric-slider.js

We will enhance the slider questions soon and take this into account.

Great, because at this moment a user cannot distinguish between touched and untouched. I saw that in the code the line that should trigger this is still there, but unfortunately it does not seem to function.

https://github.com/LimeSurvey/LimeSurvey/blob/master/assets/packages/questions/numeric-slider/scripts/numeric-slider.js#L64

Please put this at the highest level of the structure, so styling is most flexible.

Thanks in advance.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=27622

Release 3.14.2+180807