True Ames (U.S.) still make a slotted fin
www.trueames.com/pages/windsurfThe slotted Enduro is a favorite of mine and many of my friends, it's still very popular.
People started poo-pooing slotted fins long ago but the true believers are still around. The thing about slotted fins is that they have no red line, in other words, there is no limit on which you can press the fin before it spins out. If it does, it snaps back, relieving the cavitation, almost instantly w/o needing to back off on your pressure.
Many of my friends who don't like them say they are too slow. Those of us that are using them are very hard to beat, they aren't slow, we use them in the ocean and on waves.
Ever do a face plant on the face of a wave, in the middle of a hard bottom turn, because the board spun out?
That doesn't happen when using a slot.