I'm pretty certain it was supported, if you look at http://api.jquery.com/animate/#per-property-easing, those arrays of ["toggle", "swing"] are no longer functioning.
The broken functionality is caused because on line 192 of animate (https://github.com/jquery/jquery/blob/master/src/effects.js#L194), there's a check to see if the value from the options set is an array, and it then uses the second item as an easing method.. unfortunately, it never resets the value to the first item, leading to jQuery attempting to animate say, opacity, to an array value.
The code was broken in change 7666c3ef, you can see the old code before that, (https://github.com/jquery/jquery/blob/eccf15be6cfee8b567785ba29cc8841b1b0c2ef1/src/effects.js#L186) which appropriately sets the value. Without having further inspected the problem, it may be possible to fix just by reverting that one line.
I'll write together a test case to demonstrate the issue.