Update - June 13 E3 2021
Turtle Rock announced Back 4 Blood is, in fact, coming to Game Pass on launch day, October 12. It's still releasing on other platforms, but Game Pass subscribers can pick it up for absolutely free.
According to a YouTube description for a livestream (spotted on Reddit)that accidentally went live early, Back 4 Blood will be "coming to Xbox Game Pass at launch via Xbox Series X|S and Xbox One, as well as Windows 10 PC."
The video has now been set to private, but WB Interactive will hold a PvP showcase for the game after the Xbox and Bethesda Games Showcase on Sunday, so we could see the announcement made at either of these shows.
Do bear in mind, however, that this may have been an error so don't get too excited just yet.
An open beta for the game will also begin on August 5, 2021 for those that have pre-ordered the title, with everyone else getting access on August 12. That beta will also be cross-play with the goal to have this feature in the game at launch.
You can stay up to date with all the news about Back 4 Blood here in our hub. We predicted some announcements that we could see from Xbox Game Pass at Xbox's E3 show last week, which included Back 4 Blood coming to the service.
To stay up to date with all our E3 2021 coverage be sure to keep an eye on our hub.