It's amazingly difficult to get away from. For those of us who had Yahoo email accounts, trying to get people to stop emailing them is a slow drawn out process. My wife and I switched to Gmail accounts at least 5 years ago. We still get friends and relatives sending to our Yahoo accounts. This, despite multiple requests to stop.
People seem to either reply all on old messages whenever they want to contact us or copy/paste email addresses from old messages, like invitations for Christmas parties, when they contact us. Either way we can't get away from Yahoo.
The other issue is companies using email addresses as user names. Just a few days ago, I went through Yelp to order a food delivery fulfilled by Grubhub. The order confirmation listed an old Yahoo account as the contact email. This was courtesy of Yelp using the Yahoo account as my login. Yelp provides a way to add an email address and make it the primary. Not all businesses do.
If I could snap my fingers and be done with Yahoo, I'd do it. I started to move us from it when Marissa Meyer was still CEO.