No video footage survived of the Slamfest '99 Super Smash Bros

The year was 1999 and recording was by no means as simplified and widespread as it is today, but Nintendo's Super Smash Bros. 64 promotional event in Las Vegas was indeed recorded, in fact, it was actually broadcasted live

Super Smash Bros. - SmashWiki, the Super Smash Bros. wiki

I found photos in my old computer of the making of/rehearsals of Slamfest 99! : r/smashbros

Super Smash Bros. (1999) - MobyGames

EventHubs fighting game news and tournament results

New Photos from the Lost Super Smash Bros. Slamfest '99 Event Have Emerged : r/nonmurdermysteries

Smash Bros. news, videos, tournament results, streams and more

Super Smash Bros., Smashpedia

Slamfest '99 aka Super Smash Bros. LIVE (lost livestream of promotional event for Nintendo 64 crossover fighting game; 1999) - The Lost Media Wiki

Sweaty Smash Players

Super Smash Bros. (video game) - Wikipedia