Just got an email saying I have to “update my Yahoo account by 03/11/21” which is TOMORROW.  Is this legit, and what does it mean?

SCAM . . . . . . . . . . .

It is a scam to get your password.

This is nothing more than an outright scam to the third degree. Scam messages that request you to "upgrade" your email account must be ignored and deleted immediately. Never click on the links or attachments and give your personal information to the scammers or you can risk your account being hacked. Remember Yahoo would never send you unsolicited emails asking for your personal information.

Who the F cares? If you still have a Yahoo email 20 years after everyone else gave them up then you are a moron and deserve to lose it. 

That is a phishing scam. Ignore and delete it. This is how to identify legitimate Yahoo websites, requests, and communications: https://help.yahoo.com/kb/SLN2091.html

So obvious a scam.  Easy to spot - Strange sending address, and they don't address you by name - because they don't know it; Yahoo would address you by name in any personal messages...

No, it's (obviously) not legit.

It's a scam                     .

if that's not your ISP, it's bogus. if it is your ISP, go to their official website and update the account. 

It's a scam. Those emails DO NOT COME From Yahoo or Verizon. This has been going on FOR YEARS.