I'm an ESO+ member and when I log in the game is telling me I have to purchase an update to have access to Summerset. Is this accurate?
I can confirm that ESO Plus gave me access to Morrowind - which was separated into a DLC, the part I wanted now, and the Warden Class, which I might want later.
So if you wait until the next chapter, the same may well happen with Summerset.