I seem to remember in the late 90's/early 00's there was a possibility with future draft picks acquired from other teams, to roll them over.

 For example let's use this year's Dallas pick that's top 7 protected through 2020.  Say that pick falls at #17 in this draft.  In years past it was possible for the team with the pick (in this case, us, the Celtics) to choose to roll over that pick instead of using it this season.  In other words instaed of just having the #17 pick because it fell out of the range of protection Dallas put on it when they traded it to us, the Celtics could choose to let Dallas keep their pick this year, and instead could have the option of taking the pick at any year in the future its owed to them, adhering to the protections placed on it.

It's tough to explain but I'm sure someone out there knows what I'm talking about.  My question though: is this not an option anymore?  Are teams not allowed to roll picks forward?  Was it something that had to be agreed to in the trade itself?  Or do GMs just always see greater value in a pick they actually have now instead of choosing to roll the pick over and wait another year to maybe improve its stock?

Obviously I ask because with so many picks, I think it would make sense, especially if we don't find a trade partner for them this year, to roll them forward for a few years to see if the draft slot improves on some of them that we received from veteran teams who may implode in the next few years (Dallas, Memphis).  Just a thought, but like I said, it was so commonplace about 10-15 years ago, I'm just curious as to why it never seems to happen anymore.  Especially since it would seem to be possible with a defined list of declining restrictions as years go by on almost any pick traded anymore.  Thoughts?