Chapters (like Necrom) are never included in ESO+.
The Q1, Q3, and Q4 DLCs are included in the subscription. Although this year they've announced they aren't doing a Q3 DLC, and instead of the Q4 DLC being a new zone as per usual, it's a new endless dungeon instead. Just so you're informed on the year ahead before purchasing things.
I believe chapters are included in ESO+, but not until the following year, when the next chapter is released.
The zone from the Chapter is included in ESO Plus the following year, not the Chapter. Where this becomes important is with non-zone things like new classes. Necromancer is sold separately and does not come with the ESO Plus access to the Northern Elsweyr zone. I expect Arcanist will be the same way.