No, it is not phishing. Just very poor practice on ESO's part.
I had immediately pressed the spam button on an out of the ordinary email this morning that said my ESO Plus membership has expired. Well ya, I shackled my crafting bag effective at the beginning of last month. Weird they would send this now. The thing that got me to reflexively press the spam button was hovering on the link in "Please click this link to set up a new membership..." resolved the address as u29826878.ct.sendgrid.net/ls/click?upn=*** ton of tracking info
Why would anyone click on a link to the account management page that doesn't take you directly to account.elderscrollsonline.com. That is just stupid. After thinking about it I went back and look at the email I received immediately after cancelling plus and it used the same sendgrid.net domain. So today's email maybe, possibly wasn't phishing.
Please ESO update your email practices not to mimic phishing attacks mimicking actual ESO emails. At the very least it may keep your messages from being marked and automatically deposited directly into our spam folders. Or getting reported as phishing and your emails never even being routed to the end user account at all.