I'm curious as to when a "promise" becomes a legal requirement.I think they're legally required to at this point, since they promised a roadmap of content. Past that point though...
For example, Anthem promised at least 3 "Acts", but only delivered one before ceasing development.
Concord got away with it by giving everyone a full-refund.
I'd wager that Suicide Squad already had most of the post-launch content completed - I think all of the post-launch characters were leaked just after launch. And as for the costs, I can't imagine it is too expensive to keep a server online for <1000 people, and a skeleton crew to make sure that there is nothing gamebreaking.
I think it mostly comes down to following through with your promises. Live service games are sold based on the prospect of post-launch support. if you say that you will support the game for at least a year, and then ditch it after the first month, who is going to show up for your next Live Service experiment?