So I feel like different folks are saying different things, but if you only own the core game are you not able to earn rare/legendary engrams anymore? How are you supposed to level up?
There's a thing called Strike Playlist in the game. It has different tiers for different level versions, but basically, it puts you into a queue to be matched to a random instanced dungeon.
Pre-expansion, the highest level of this was Vanguard Tiger, which scaled the content to level 24 and gives bonus engrams (unidentified items) on completion - this is in addition to what may drop during the mission itself.
With the expansion, a new playlist called Vanguard Roc was introduced, which matches content to level 26. This new playlist rewards bonus engrams, which are no longer rewarded from Vanguard Tiger.
So you'll still get engrams, but you just don't get the bonus engrams you used to get from Tiger, as that reward has been moved up a tier, to the tier that requires DLC to play. This part is kind of scummy.
Additionally, a lot of people are loudly miffed about new content being added to the Weekly Heroic and Nightfall events, which are both great ways to get gear without raiding, but what's actually happening is that players simply can't play content they didn't purchase. Realistically, it's pretty reasonable to think that new content would be added to a weekly rotation, even if you didn't purchase it yourself.