Pretty sure Stormreaver has always been meant for Electrocute with Storm Totems and Savagery filler (back in the day it was taken for Tenacity of the Boar/Might of the Bear), for example jajaja’s original Warder which the Mythical was built on or mad_lee’s take on it.
Compare the damage on it to other similar weapons:
Infernal Brimstone
Guillotine
Rancor
Lightning Cadence support got added after the Mythical released when attempts to change Discord to Physical->Lightning were rejected by the community, so to keep it alive in some way, the support was moved to a handful of items instead like the axe and Exonerator.