No, the cleaning of the cache is not linked to the debug mode.
When debug mode is on, the asset manager is not used, so devs just need to refresh their browser cache to update the CSS/JS.
Else, the asset manager is on, working like a charm since months, so people don't have to worry about it at all, nor to understand how it works.
The effect of using the asset manager is that final user (participant of a survey) do not need to refresh the cache of their browser to have the last version of the CSS/JS. Before, in this forum, you spent quite a lot of time explaining people they should say to their participants to update their cache (which is a pretty complex operation on a mobile phone without cleaning all the cache of the web browser).
About purging the asset directory, it's done each time we release a new version of asset (so basically, at each release). The code is here:
github.com/LimeSurvey/LimeSurvey/blob/ma...teForm.php#L380-L389
It could be easily moved to a different function, and then we could give users a control to do it when they want, but it's just totally useless. You never have to do it. It will never fix a problem. We're doing it only to avoid the folder to become huge after few months.
you can read the Yii documentation about it:
Many newcomers ask: "What do we do with the assets folder?", and the answer is "Mostly nothing".
www.yiiframework.com/wiki/148/understanding-assets/#hh4
