Brehs. There’s only alternate timelines if the stones are removed from that timeline. I remember the ancient one saying they control the flow of time. That’s why they needed to to return them right before they were taken. To make sure you don’t create a alternate one.
Old Cap existed with the young Cap. It’s how he was able to show up at the end. He didn’t cross dimensions.
So what about Gamora killing her old self, and Tony dusting Thanos and his crew? Doing that hasn't changed the present. So now there is a timeline where the events of Infinity War don't happen.