In Destiny 2, you're initially limited to the game's newest subclasses for story reasons we won't spoil. The others can be unlocked later, however, so if you’re wondering when you can get your hands on your beloved Nightstalker again, or start throwing some more Hammers of Sol, here’s when and how.
For a full rundown of all nine subclasses, check out our Destiny 2 class guide.
You unlock your second and third subclasses through items called Class Relics, which will start dropping when you reach certain level thresholds. Many Redditors on this thread report receiving their first at level seven, and it looks like the second comes at level 14 or 15. OP ShadowRock9 reports that the Relics can drop from any chest, and that, once you're at the level threshold, the drop rate is pretty high.
The next step is to ‘charge’ the Class Relic by killing things. The Milestone prompt suggests you do this in public events, but ShadowRock9 says this only speeds things up - any kills in PvE or even PvP will raise the completion percentage. So really, all you need to do is play the game.
Once you hit 100%, a new mission will unlock. It looks like this is similar for all subclasses in that it's always set at the Shard of the Traveler on Earth, and involves wasting hordes of enemies with your new Super, which you can fast-charge by passing through pools of Light. Complete this mission to unlock your new subclass.
Hunters start as Arcstriders, unlock Gunslingers first, and Nightstalkers last. Titans start as Sentinels, unlock Strikers first, and Sunbreakers last. Warlocks start as Dawnblades, unlock Voidwalkers first, and Stormcallers last.
Here's ShadowRock's video demonstration.