Didn't see any thread so far.
GamesRadar reports the following:
- Xbox (confirmed partner, showcase expected)
- Nintendo (confirmed partner, showcase expected)
- Future Games Show (showcase confirmed for June 13)
- PC Gaming Show (showcase confirmed for June 13)
- Square Enix (confirmed partner, showcase expected)
- Ubisoft (showcase confirmed for June 12)
- T2 (confirmed partner)
- Warner Bros (confirmed partner, showcase expected)
- Koch Media (confirmed partner)
- Capcom (confirmed partner)
- Activision (confirmed partner)
- Bandai Namco (confirmed partner)
- Sega (confirmed partner)
- Gearbox (confirmed partner)
Bit of a switch this year as apart from being a digital only event, the event itself runs from
Saturday June 12th to Tuesday June 15th 2021. Which I think is fantastic. Hard to stay up late watching the conferences during the week with work.
One of the heavily rumored / expected titles is Bethesda's Starfield and it's confirmed to NOT be released on PS5 whenever it does come out.

Official E3 Website:
https://e3expo.com/