Xbox refused cross-play back when the 360 was dominating the Ps3 and Sony was doing the same when the PS4 was but I thought I heard that PlayStation decided to support cross-play across other games after Fortnite.
Also it's not Playstation + but Playstation network for Sony. Playstation + is a service to get free games and access the multiplayer of most games but not all. Some games like Fortnite don't require you to own a playstation + subscription to play online.
Anyway, xbox live and playstation network in no way prevent cross-play nor cross-saves. For cross-play and cross-saving the actual game just has to let you log-in into a specific game account.
@Bleu42 said : "For D4 I believe they've said cross play / platform would be nice but no plans, so I'm assuming no."
One of the panel recap point on the main page says this : "Crossplay between consoles is something the team is aware of and they’re currently exploring options, but can’t give any answers at this time."
So that sounds like a possibility more than Bleu42 makes it look like. But it only mentions cross-consoles and not PC and cross-play doesn't mean cross-saves.