So, let's get back in time when ETH hard forked.
If you had 10 ETH on lets say poloniex, after hard fork you had 10 ETH and 10 ETC (correct me if I'm wrong).
Is this going to happen as well if BTC hard fork occure?
I think it will be the same for bitcoin, you would end up with BTC and BTCC/BTU/whatever.
If you leave your coins on crypto exchanges though, they MIGHT only give you access to one version of bitcoin. So it's never a good idea to leave them on exchanges. Keep them on wallets whereas you have access to the private keys.