After some amount of poking around, I've found exactly one case where that happened (https://en.wikipedia.org/wiki/Lavabit https://en.wikipedia.org/wiki/Lavabit

After some amount of poking around, I've found exactly one case where that happened (https://en.wikipedia.org/wiki/Lavabit https://en.wikipedia.org/wiki/Lavabit in connection with Snowden). Law enforcement might subpoena, say, search result records in connection with a specific crime (certain keywords + location + timeframe), but larger companies with the resources would absolutely have the ability and incentive to fight something as overly broad as requesting SSL keys.