NetApp is changing the way they handle volume autosize - the Deprecated notice is simply indicating that the change will be required in a future release.
Looking back at autosize, first we had grow only, with 8.2 they added shrink. Now they are moving away from a single static increment by which to autosize. It has traditionally been the case where you could specify either a fixed increment (like 100GB) or a fixed percentage increment (5%). However, as volumes get big, neither of these might be best. The fixed size might not even be enough, at scale, to get beyond the autosize threshold, and the system ends up pushing multiple autosizes through in sequence. A fixed percentage based adjustment generates exponential size jumps as the volume grows or shrinks.
CDot 8.2 also let's you specify directly, for each volume, the percent full/empty thresholds at which autosize growth/shrinking should occur per volume.
8.3 still has all of this and will keep the traditional autosize functionality throughout its patch cycle. However, in a future release (8.4 or later), NetApp will remove the specific increment amount controls from the user and instead calculate the size adjustment automatically, when needed, based on the threshold settings and perhaps other factors. The intent, I think, is to eliminate the edge cases in the current mechanism so that an autosize change happens once per threshold event (the size increment too small case) and is able to succeed when there is room to grow/shrink (the exponential percentage case). It is a logical change to make.
These types of functional and operational changes have been made throughout the 8.x line.
Hope this helps!