Not sure about SMS mail, but Yahoo has opened up IMAP for quite a while -- about 5 years or so.
Mail Plus is gone and replace by "Yahoo Ad-Free Mail" which gives:
No text or graphical ads in Yahoo Mail on desktop browsers.
The account will not go inactive.
"The Yahoo Mail Plus premium service has been replaced by Ad Free Mail. If you're still subscribed to your original Yahoo Mail Plus account, here's what to expect when it automatically renews.
Once your Yahoo Mail Plus account has been renewed, it will transition to Yahoo Ad Free Mail. You'll still enjoy your email without ads, but at the price you've always paid (new Yahoo Ad Free Mail accounts cost $49.99 / year).
You don't have to do anything. As long as your account with Yahoo is in good standing, you'll continue to renew for $19.99 each year on your anniversary date."
Anyway, I don't think they put signature ads in SMTP mail, and adblock works pretty well for Yahoo free (old basic HTML 4 interface).
I get the occasional "can't log in" on mobile and Apple Mail, but feh... works well enough.