Do I get next season pass from DLC’s

You described the differences pretty well.

In Destiny 1, shaders were normal items - they were not consumable. There was only one shader slot, and it applied to all your armor (weapons were not affected (related, most exotic weapons had one or two ornaments that worked similar to shaders, but were consumable)). You could change the shader at will, and the shader would not be destroyed. Later on in Destiny 1's development they added a shader kiosk where you could get a new copy of any shader you have ever found which would allow you to effectively use the same shader across all your characters.

In Destiny 2, shaders are now applied per item and can be applied to weapons, sparrows, ships, and ghosts in addition to armor. Shaders are also consumable - once you've applied a shader, you can't use it again. If you replace it with a different shader, the first shader is gone. When a shader drops, you typically get a few copies of it (I've gotten 3 at once, not sure if that is guaranteed).

The main issue some members of the community have with the new system is that in addition to being consumable (which means you tend to think about applying a shader, since you can't use it again), is that you can buy them with real money via the Eververse store. The conspiracy theory is that shaders were made consumable so that Bungie would make more money on microtransactions.

This has been covered off in this weeks (6/13/2019) This Week At Bungie.

Specifically,

Forsaken and New Light
Q: Will Destiny 2: New Light be its own separate game/file?
A: No. All Destiny 2 players will be automatically converted to New Light and will maintain all of their prior purchases and entitlements.

Q: What Forsaken and Forsaken Annual Pass content will be included in New Light?
A: Forsaken content available in Destiny 2: New Light includes: Free roam on all destinations, Strikes playlists, Crucible playlists, Gambit and Gambit Prime playlists, Select Annual Pass content

Q: What Forsaken and Forsaken Annual Pass content will NOT be included in New Light?
A: Forsaken content available only with purchase includes: The Forsaken campaign, Year 2 raids, and dungeons Exotic quests

So you'll only need to purchase Forsaken if you'd like to play the Forsaken campaign, dungeons and acquire Forsaken exotic items.