When DLC is introduced into a game, it will inevitably split an audience somewhat. The game will move on with the new content, and certain activities will be locked away. Of course, for new content this makes sense but certainly, it can affect older modes that used to be in the base game. That has seemingly hit Destiny 2 with the release of Curse of Osiris.
As Reddit points out, the DLC raises the power level to 335, and thus in order to keep the hardest version of the Raid exclusively for the higher levels, the power requirement has now been raised to higher than is accessible to people without the DLC. The logic makes sense, but in reality, it means not only is not having the DLC keeping you from new content, it is keeping you from old content, too. Understandably, that hasn't gone down well.
Right now, it's not clear what this means for the future. The game has certainly been the focus of continued criticism, with several changes made recently to right community concerns about the endgame. It seems they've now stepped into a new controversy. At the same time, while it is rubbish that old content is being gated off, the prestige Raid was a fairly niche endeavour and this likely won't affect all that many players. I think the principle of it is more the problem here.