The most common sense of “forthcoming”, as far as I'm aware, is used to describe a person who is willing to divulge information, or open and willing to talk. I would describe the opposite sort of person as “tight‑lipped” or “cagey”.
Another sense of “forthcoming” is used to describe events that are planned for or about to happen in the near future. There's not really a good antonym for that sense of the word, because it contains within it the concept of imminence, and the time of the future. This means the opposite might be something that's a long ways off (but still to come), or the opposite might be something that just happened.
“Bygone” refers to events that are long past, so that could also be considered an antonym for this sense.
The other sense of “forthcoming” I know is used to describe necessities which are “available when wanted or needed”. This one's a little tricky as well, because most people would either simply say “not forthcoming” or use the opportunity to indicate the reason for the unavailability of whatever it was. There are also specific words like “frozen” (used to describe assets that have been "prevented from being used for a period of time") or “waylaid” (used to describe a shipment that has been ambushed) which might fit best, depending on the situation.
You can read more here, which seems like a pretty good place to find antonyms in the future.