If you don't own Forsaken you will need to either buy it or wait for the "Forsaken Pack" which, from what I understood, will contain all the Forsaken content that will NOT be removed with this years DCV update. S dungeon, raid, exotic, and middles tree supers.
For stasis just grab Beyond Light should be on sale ATM.
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To add on to what ThatGuy33133 has said. The Forsaken pack will be available later, it will only include access to what is left of the Forsaken content after the vaulting. The Legendary edition is a bundle that includes Forsaken, Shadowkeep, and Beyond Light. If you want to replicate the access that is leaving game pass, this is the bundle that will do that. It basically includes everything that is leaving the pass.
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The Forsaken pack is just the Forsaken content that is not getting vaulted, such as Last Wish, Shattered Throne, Middle tree supers, etc. This will be your best choice if you are only missing some content. I didn't see it in the Microsoft store but apparently there will be a separate bundle that will include the Forsaken Pack and then all the DLC's up to Beyond Light. This will catch up players that only played with Game Pass and do not own any previous DLC's. I wasn't able to locate this bundle so take it with a big -blam!- grain of salt.