Bungie has spelled out, in intricate detail, exactly what content is going to the Destiny Content Vault (DCV) when Destiny 2: Beyond Light and Year 4 releases in November. The short version is that Io, Titan, Mars, Mercury, and Leviathan, and all their associated PvE content — strikes, raids, and general exploration — will be shuttered. PvP modes (Crucible and Gambit) will be less impacted, leaving a “best of” list available while some of their maps get transferred to the DCV.
Many quests for exotic items will also become unavailable at this time, but those that do will still be available via a new Memorial kiosk in the Tower. Bungie will reveal exactly how they may be acquired at a later date. If you already own the items, they will be unaffected.
This would seem to leave a relatively small amount of content for new players to explore, particularly since the early campaigns — Red War, Curse of Osiris, and Warmind — which were touted as all being available when D2 went free-to-play, will no longer be available. They will be “replaced” with a new new player experience centered around the Cosmodrome, but it’s unlikely that will sufficiently replace the vast amount of content that was previously offered.