The issue? Any gear above 300 (305 with Mods equipped) requires the player to be level 21 to equip. Curse of Osiris also raises the level cap from 20 to 25, so those who do not own the DLC can not physically be high enough Power to participate in the activities.
The key point here is that this content has been locked from players that have already paid for it.
This issue first appeared when Destiny 2 players realized that, after Bungie raised the recommended power level to 330 for the Prestige Nightfall and Prestige Raid, players without the DLC would be unable to complete them. What seems a bit less natural, however, is the fact that the Prestige activities from the original game have had their power levels bumped, meaning you can't access them until you've grinded out to the new cap in the DLC. But the problem didn't end there. If you bought the new DLC or the expansion pass then I would not be surprised if you felt that the expansion was short or that it lacked content. The Strike doesn't say that it is PL 330 when hovering over it in the Destination Map, but players will soon find that out when the enemies decimate their fireteam. When those Strikes are that week's Nightfall, those without the DLC can not play it.
To take into account the new levels the Prestige versions of the Leviathan Raid and Nightfall Strike, they have had their requirement levels pushed up to 330. Keep in mind that at the end of the day Bungie is in the business in order to make money and if this system is working for them then this is what players are going to get.