Last Blade 2 rollback netcode beta is live on Steam as a free update. PS4 will also get updates with rollback netcode after the Steam beta / official update.
Vita version update will come much later, so cross-play with PS4 will be suspended until then.
So, if you're interested in participating, right click on THE LAST BLADE 2 in your library and select Properties, then go to the Beta tab and enter the Beta Access Code "StayAtHome2020" (without the quotes). Then click "Check Code". You'll see "deliverable - 2020 rollback update" appear above in the "Select the beta you would like to opt into" list.