Never heard any exchanges that have this feature and I think there is no exchange that will add this function because more traders preferred to withdraw their coins than holding their coins on the exchange site. There are chances that the exchange site suddenly shut down without giving alert or notification to traders. So it is not a safe function if they have this feature.
If you want to hold your coins for a long time you need to train your self to not sell your coins in the bear market this is the only safe way compared to stay your coins too long on exchanges.
Or you can find a trusted friend who can hold your coins for a long time and let him know the date where you can get your coins back. This is just my suggestion for your coins safety.
Oh didn't consider that, you are correct, people prefer to withdraw money and don't leave it on exchange but still there are some people who somehow find hard to manage own offline wallet and store btc on exchanges.
If you ask friend for that, you know some people who can't control their actions well, maybe will require those stored money and if friend didn't give back, it will cause conflict and so on.
Well, I understand there isn't such function, I thought some of you knew any exchange that had it.