https://www.gamespot.com/articles/destiny-2-curse-of-osiris-locks-non-dlc-owners-out/1100-6455455/

As was the case with The Dark Below DLC for the original Destiny, Bungie has effectively locked Destiny 2 owners out of some activities they previously had access to if they have not purchased Curse of Osiris. The expansion launched earlier this week and, among many other things, increased the level and Power level caps, resulting in a corresponding increase to level requirements for select modes. As a result, those who have not purchased Curse of Osiris suddenly find themselves unable to take part in things they could play just a few days ago.