Should I sell my remaining Rift Keystone fragments?
Since the 2.3.0 patch, any rift keystones now have a message showing in the tooltip saying "This is useless. Perhaps you should sell it."
Given that they are no longer used to open the Nephalem rifts, is that indeed the case, or is there still some hidden use for them?
Not that I know of. The purpose for the devs to change the Obelisk to not use key stones for rifts was to change the dynamic between bounties and rifts. It's doubtful that these have anymore use, I've gotten rid of my own since they take up stash space.