Setting up automatic forwarding is a common practice for users who either use more than one email address or want to make the transition to a new provider. By disabling this feature, Yahoo has put up a roadblock which makes moving to a new email provider difficult.
"This feature is under development. While we work to improve it, we've temporarily disabled the ability to turn on Mail Forwarding for new forwarding addresses. If you've already enabled Mail Forwarding in the past, your email will continue to forward to the address you previously configured."
If you have already set up email forwarding in the past, you should not have any issues with continuing to use this feature. Instead, the "development" of the feature is likely to affect those wishing to leave Yahoo now -- and potentially because of the company recently admitting that a cyberattack in 2014 led to information leaks affecting at least 500 million accounts.
"We're working to get auto-forward back up and running as soon as possible because we know how useful it can be to our users. The feature was temporary disabled as part of previously planned maintenance to improve its functionality between a user's various accounts."
Until this feature is no longer "under development" -- which is somewhat ludicrous considering how it is a basic service offered by almost every other provider out there and maintenance wouldn't necessarily only affect users now attempting to set it up -- there are ways to circumvent the block.
While not as efficient, you could, for example, set up a vacation message containing your new email address.
2016: The best high-end tablets for business users