You’ll lose any My Yahoo personalized settings when you delete your account. Additionally, Yahoo offers quite a few services linked to your Yahoo account, so you will lose access to them when you delete Yahoo, including:
Emails
Calendars
Stored investing portfolios
Fantasy sports team and league information
However, for a site like Rivals.com, which Yahoo owns, you can create a Rivals-only account, so you don’t need a Yahoo account to access your Rivals subscription.
Over the past decade, Yahoo shut down many of its services linked to Yahoo accounts, such as Yahoo Messenger. It sold sites like Flickr as well. So if you used services like these in the past, they’re no longer part of your Yahoo account.
The best advice is to determine if you are using your Yahoo account information to access any sites and services you use regularly. If so, update your account settings to use another active email address. If not, losing information when you delete the account should not be a problem.