When E3 2021 was first announced, there were many absentees on the original exhibitor list. Now the list of participating companies has been updated to include a total of 43 developers and publishers, including many of the most notable omissions from the first list.
Nintendo, Xbox, Capcom, Konami, Ubisoft, Take-Two Interactive, Warner Bros. Games, and Koch Media were all confirmed to be taking part during the initial announcement. Square Enix then confirmed their participation during an interview. The list, which has since been removed from the official E3 website, now also includes notable developers and publishers like Activision Blizzard, Bandai Namco, Deep Silver, SEGA, and THQ Nordic.
E3 2021 Participating Companies

Activision Publishing, Inc.
Amazon Game Studios
Bad Button Studio
BANDAI NAMCO Entertainment America Inc.
Bethesda Softworks Capcom USA, Inc.
Click Entertainment
DvG
Deep Silver, Inc.
Epic Games Inc.
Funcom
GAMELOFT
GFUEL
GTR Simulator
GungHo Entertainment Online America
HORI USA
Hyperkin, Inc.
Kalypso Media Group
KontrolFreek
LGA Enterprises
Limited Run Games
NCSoft
nDreams
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Nintendo of America Inc.
NVIDIA
Oculus from Facebook
PureArts
RDS Industries Inc.
Rebellion
SEGA
Square Enix, Inc.
Take-Two Interactive Software
Tastemakers
TECHNISPORT
THQ Nordic
Ubisoft Entertainment SA
UCC Distributing Inc
UnnamedVR by Paracosma
Warner Bros. Games
Xbox
XSEED Games
Xsolla

There are still some notable absences from that list. Sony didn?t participate...