It most certainly does! You have to make sure the "also resize enclosing part" is checked...
The thing about sliding is that you can't expect a line or such to slide up if there is nothing above it. The reductions in space only occur when the objects take up less space because they are not populated with data to the depth allowed. The savings on space only represent the amount the things above are reduced by.
Often sliding doesn't work because you haven't told all the objects below to slide up...
Of course sliding only shows in Preview.
Attached is a little sampler
Sliding.fp7.zip 6.6 K
A fresh day revealed a text field (label) that did not have “Also reduce enclosing part” checked. All works as I remember.