Yahoo Briefcase was a service that allowed people using Yahoo to store up to 25 megabytes of data, for "free". As the 25mb (not gigabytes, and not 250mb, but twenty-five megabytes) indicates, this service stored a very large amount of data, therefore requiring Yahoo to reduce the amount of data allowed stored by account. Why Yahoo suddenly decided to stop supporting this service is unknown, but the warning time was roughly 60 days, which is long by Yahoo standards but hardly ideal for a service up for nearly a decade.
Any attempt to access http://briefcase.yahoo.com results in being redirected to the respective Yahoo homepage for your country.
Here is a copy of the letter sent out towards the end of the 60 day period: