LogFAQs > #978958071

https://www.starwars.com/databank/mace-windu

When Anakin discovered Palpatine was the Sith Lord Darth Sidious, Mace knew the future of the Jedi Order was in grave danger, and sought to arrest the Supreme Chancellor. Mace seemed to defeat Sidious and raised his lightsaber to strike down the Sith Lord, but a desperate Anakin ignited his own weapon and severed Maces forearm. Sidious then blasted Mace with Force lightning, sending the great Jedi champion plummeting to his death.

https://starwars.fandom.com/wiki/Mace_Windu#Duel_with_Darth_Sidious

Windu, whose original goal was to put the Chancellor on trial, concluded that Sidious was too dangerous to be left alive due to his control over the Senate and the courts. But before Windu could deliver the killing strike, Skywalker betrayed Windu by severing his right hand, enabling Sidious to unleash a torrent of Force lightning that sent the Jedi Order's great champion plummeting to his death.

Both the official site and the Wookiepedia site list it as such. So officially dead. Doesn't mean they might not decide he survived for some pre Death Star spinoff, but until they do, he went splat on impact.