Maintaining clearances in HV MOSFET footprints

I've wondered about this myself.

What I ended up doing for a 230VAC mains project: I created a custom footprint for the TO-220 part, where I moved the outer two legs out to approximately a 4mm pitch (up from 2.54), and the center leg forward. Then I routed slots in the PCB between the pads for additional creepage distance.

This won't change the creepage on the part body itself, but at least it will get the best possible isolation on the board:

You'll need to bend the TO-220 legs when assembling. This is perfectly doable for small manual assembly runs, but I suspect it doesn't lend itself well to large-scale production.

The better solution for high voltages is probably to use a larger part, like a TO-247, which naturally comes with a 5.44mm pitch. That should easily allow routed slots between the pads:

(image from Infineon)