SUP Foil: kite reel leash attached to a waist belt. Looks dorky as hell but super effective. No tangling, no drag, minimum snap back on bigger wave wipe outs. I originally used sup coil leash, but it quickly became half uncoiled from those bigger waves, and then it is a big tangled ball/anchor to drag around.
Surf foil: I haven't experimented with reel leash yet. I found a longboard leash tended to wrap around wings too much while waiting for waves. I am now using a thin 6' comp leash, seems much better.
Not using a leash seems pretty sketchy if there's anyone towards shore of you. A foiler maiming a civilian = BAN.
And a warning on wing foil: board leash is a MUST, from personal experience. A floaty sup with a foil will float away from you faster than you can swim, even in light wind. <ugh>