Yahoo email addresses on your list that haven't opened an email from you in 12 months are very likely to be among those released.
Other people who are
not on your list are very likely to claim these addresses. If the addresses are still on your list, the new owners will start getting your newsletters.
The new owners won't expect your emails. They will either unsubscribe or mark you as spam. The latter can negatively affect whether ISPs such as Gmail and Outlook accept your emails.
If the released addresses go unclaimed, your emails to those addresses will bounce. This can also impact your deliverability.