In my Jenkins, the first thing I normally do when I create or edit a job configuration - check if workspace cleanup is enabled. 99% of the jobs must have clean up enabled after the build, else if working with larger tasks that demand disk space, it is inevitably used up and requires either manual clean up or additional automated mechanisms to do that.
I imagine this proposal might be controversial, since a lot existing projects rely on the way things currently work, but I'm still putting this here up for opinions and discussions. In my opinion, having any task perform clean build and being detached from environment is a good idea which makes maintenance a little bit easier, even if we're not concerned with disk space. Guess a better solution for this is containerization, but Jenkins could prod his users towards 'clean' projects by default.