How to correct this sentence？
Although this phenomenon was a tradition in ancient times, it isn't a good choice.

Question

How to correct this sentence？
Although this phenomenon was a tradition in ancient  times, it isn't a good choice.

Answer

It's OK as it is. 
Ideally it wants something more, to explain why it isn't any more.

Answer

As Gary said, there must be reason it is no longer a good choice.
The phenomenon, or practice, performed in ancient times, may have been unsafe, or inappropriate by today's standards, therefore it would be a bad idea to continue doing it.