Depends on packaging material, and project regulations.
IR21xx for example are available in SOIC-8 and claim 600V handling. Have also seen TSSOP/MSOP hot swap / load controllers rated say 100V.
IC packaging resin may be considered to have a better than usual CTI (comparative tracking index); it's surprisingly rich in silica. Perhaps if arcing occurs, the organic binder burns away as much as it carbonizes?
If it matters, you'd likely want to either:
a) ask the manufacturer about the part ratings, standards met, and packaging material;
ask the certification agency (i.e. UL, etc.) what they think about it;
go back and forth until a suitable device has been selected; or,
b) just get the SOIC-14 part with gaps (NC pins) included. Or parts with the pins removed (more common in offline SMPS controllers, I would say?).
Tim