Dark Souls Remastered is coming to PS4, Xbox One, Switch, and PC, but if you were hoping for any cross-play between the platforms you're in for some disappointment. Cross-play for Dark Souls Remastered didn't even reach the "rumored" stage before it was shot down by Bandai Namco. Unfortunately, cross-play, in general, is a feature that players love, but hardware manufacturers are reluctant to embrace.
Multiplayer for titles being locked to a specific platform has been a quandary since online play on consoles really took off during the years of the Xbox 360 and PlayStation 3. Pledging to a console wasn't a matter of fanboyism anymore, you just had to buy the console your friends had if you wanted to play as a group. We've seen the slow emergence of cross-platform play over the last few years, but it seems like we're far from it being a universal feature. Sony, especially, seems reluctant to open their console up to interact with players from other platforms.
I suppose it depends on the game as to whether it's hard to implement cross-play multiplayer. However, we did see an incident a few months ago where Epic accidentally enabled cross-play between PS4, Xbox One, and PC for Fortnite. If it's something you can do so easily by accident, it likely isn't lack of development resources that's holding back the feature being added to more games.
Given Dark Souls' unique multiplayer system, it would have been awesome to get the opportunity to be paired with an even wider array of players with cross-platform play. However, given the popularity of the series, it's likely that Dark Souls Remastered will be a hit and you'll have plenty of chances to partake in multiplayer.
You can prepare to die when Dark Souls Remastered is released on May 25, 2018, for Nintendo Switch, PS4, Xbox One, and PC.