Square Enix has confirmed that it will be at E3 2021 in some capacity according to a recent interview.
Square Enix president Yosuke Matsuda said the following:
"We'll be announcing the rest of the line-up [later], and we're also planning to make an announcement at E3 in June, so please keep your eyes peeled,"
It's unclear if the company will be holding a full show or maybe something smaller but Square Enix is having a great year so far with the launch of Outriders and I'm sure they'll want to keep that going for as long as possible.