Is it okay not to replace the leaking Jetta heater water tank? Will it affect driving safety?

It won’t affect you, as long as the heating water valve doesn’t leak, you’ll be fine! When you don't turn on the heater, check to see if there is any water leakage. If there is no leakage, it doesn't matter. The air conditioner cannot provide heat! Yours should be a manual air conditioner, so there will be no problems and no impact on safety. Just pay more attention to whether there is a lack of coolant! Hope it's useful to you.