Submitted as a feature: bugs.limesurvey.org/view.php?id=9816
Thanks all!

holch wrote: I am not sure if this shouldn't be a core feature rather than a plugin. This is clearly something the survey engine itself should handle, I think. But then I am not a developer.

I strongly vote for making this a core feature. Plugins are nice for very special stuff but closing a survey once X completed responses were collected is a common approach at market research surveys.

It depends on the way plugins are shipped and tested. When we read the name "plugins" it can come in different forms. In Joomla there are plugins, which are always shipped with the core installation and always activated. The difference is only visible for the programmer, which needs to use a API.

When plugins means, that users need to download them and activate them, then I see a lot less testing of these features. When we look at 2.06+ with all its regressions it might be useful to encapsulate more code.
Currently 2.06+ isn't stable. The bugtracker shows a clear picture. With plugins the debugging can be quicker, because code parts can be easily switched on and off (at least for the programmer). I would suggest to get the wording and the concept clear. I would suggest to introduce coreplugins as name, for functions, which are plugins, but will be shipped activated with the core installation.

While I agree that using the term "core plugin" makes sense for such basic, non-exotic stuff I still vote to add this filter directly add the code.
It should only take some additional lines since the concept is the same and the flag for filtering ("Q") is already there.