Will adding onion ruin or enhance a chicken broth?

Onion adds flavour to any dish (other than deserts - but now that you mention it LOL)

chicken

I've never made it without onions.

Any broth / stock will be enhanced with onion.

I cook very few things without using onion at some point

while it can enhance the taste, it depends on whether you are adding it to already made broth or making it from scratch. and what you are using it for.

chicken broth traditionally includes onion in the stock.

Depends.

Classic chicken broth will include onion, carrot and celery.

When I am making broth that I will be feeding to or sharing with my dogs, I omit the onion and it’s better for the dogs to not have onion. I can always add onion to whatever I am using the broth for later.

I would say enhance

Enhance.