Game-sized Fallout 4 mod Fallout: London has been delayed indefinitely by Bethesda’s incoming next-gen update

“the unavoidable fact that it’s going to break our core systems” means that fixing everything “will take some time”. As such, no replacement launch date was given - “We might get lucky with a day-one fix, it’s unlikely, but it might be a month,” the developer said. “Soon as those fixes are done, we can put it back up again.”

Since 'indefinitely' tends to mean forever in gaming news, it's worth clarifying that the creators just can't give a date right now. They need to wait and see what needs to be updated, which makes sense.