More precisely, master links were not found on derailleur chains until the increasing number of cogs on the rear cluster meant they needed to be so narrow that they were riveted, rather than pinned, together. To open a chain lacking a master link, you use a chain tool -- the same tool you use to shorten a modern riveted chain prior to installing it with a master link. But as fietsbob notes, you need to take care not to push the pin all the way out; just far enough so that a twist of the chain will allow it to separate.