Funnily enough (no, not really), I can't delete my Yahoo! account or even set it dormant because my ISP (BT) had / has an agreement with Yahoo! to provide email for domestic users going back to the end of the 90s. Even though my primary contact registered with BT hasn't been this address for well over a decade, because it was the original I signed up with it can't be deactivated because reasons.
The "don't log in for 90 days" trick doesn't work with this account. I hadn't accessed it in five years. Fortunately I'd set all mail to be marked as spam (if it wasn't already) before I left it alone all those years and the size of that folder was astonishing.