This was confirmed by GamesIndustry.biz, who got the following statement from an Activision representative:
"While we won't have a traditional space on the show floor, we look forward to participating at E3 across a number of activities."
Activision will still have private meeting rooms at the show, and in other Activision news, we recently reported on rumblings that Call of Duty 2020 has been cancelled, with Black Ops 5 taking its place.
EA won't be on the show floor at E3 either, but that's understandable considering their presence at a separate EA Play event (which won't feature a press conference this year). Here they'll be showing off several games via livestreams at the Fanfest, starting on June 8.
Are you sad not to see Activision there?