I think at this point every Bitcoin business stores their coins in cold wallets, but they also always have a hot wallet for quick withdrawals for their customers and this is usually the target of hackers.
...
There are a few services that don´t offer a hot wallet and are even proud of that
idiosyncracy. E.g. BitMEX (probably the biggest trading site for BTC derivatives) doesn´t have
a hot wallet and withdrawals are processed manually once a day.
According to their own claims they have never lost a single Bitcoin
through hacking. Depending on the kind of service this might actually
be a good option for many other Bitcoin businesses (of course it is
not feasible for an exchange with thousands of withdrawals per day).