I haven't been able to find this a plugin effect (or hardware processor for that matter), partly because I don't know what generic name it's known by so I could search. Its purpose is to "rotate" the stereo field by an adjustable angle.
There is only one control, the angle (or α).
The outputs LOUT and ROUT are defined in terms of inputs LIN and RIN by the rotation formula:
LOUT = LIN × cos α - RIN × sin α
ROUT = LIN × sin α + RIN × cos α
When used with an angle of 45° it's essentially equivalent to a mid-side encoder.
When used on a true stereo source (e.g. from a Blumlein pair) with a varying angle it would theoretically give the impression that the sound source is rotating around the listener.
Web searching so far has turned up the names of some plugins that may have done it, but the websites/product pages are defunct or no longer maintained (e.g. roundpan).