Yahoo Mail is Yahoo's email service at https://mail.yahoo.com/.
According to https://help.yahoo.com/kb/new-mail-for-desktop/empty-mailbox-yahoo-mail-sln29338.html "If you haven't signed in to Yahoo Mail for 12 months or more, your mailbox is considered inactive. An inactive mailbox stops receiving new emails, and all mailbox contents, folders, contacts and settings are permanently deleted."
"Cloud storage" websites are known for deleting user's files after one year of not signing in or being active, but email websites are not known for this treachery; maybe Yahoo will be pioneers in this type of mass deletion. (Email messages are usually small-filesize plain text content which sometimes have one a few megabytes of attachments; small filesizes notwithstanding, Yahoo will delete it.)
The following email is suggestive of Yahoo purging user's inboxes after one month of not logging into their email account at mail.yahoo.com. Said email, from https://mail.yahoo.com/d/folders/1/messages/# (where #=one or more numbers):
(A raw/un-rendered version of this message from https://apis.mail.yahoo.com/ws/v3/mailboxes/[...] may be made available in the future.)
Oddly, the Yahoo account in question was probably last logged into 1 or 2 months before 2021-06-01, not 12 months before.
- https://twitter.com/waywardcalico/status/1403014482146824197 - title: "wayward calico cat squad on Twitter: "This morning I logged in to a Yahoo account I’ve had for 15+ years. When I logged in and got a message - due to inactivity my inbox was deleted. Every single email I ever received from my deceased father is now gone forever. Another piece of him lost to me. Thanks, @YahooCare" / Twitter"