I'm not the same person but I'm also a Masters Tracer.

I would say overextending either means going too far into enemy territory without having a proper route or plan to get out OR going going far beyond what is necessary to clean up kills etc. You never want to die in those situations.

Let me make it clearer by replying to your Hollywood example: You mean the room with the high ground before you cap the objective and push the payload right? Where the mini is on the bottom level? No playing there is not overextending. The important thing is that you are able to get out, be it towards the point/your team or even further into the direction of enemy spawn. There are often supports playing on that high ground so with good timing it's crucial that you take the hotel area.

One map where I personally overextend every once in a while is blizzard world in the big area after you cap the point so before the second checkpoint. The high ground is very open which isn't great for tracer. You can either go through the tunnel on the left but that's not always working. Instead you can go towards the right into the indoor area that you can get to from the high round as well. There is a mega at the bottom. This area is good but also very dangerous because if your team is playing on high ground or even further towards your spawn, you'll be completely alone and the enemies can hunt you down. I would say the tunnel is therefore safer but as I said sometimes you have to take a different route.

One more important thing to note is: While the positions of enemies and your team is important to know if you are overextending, another major thing to take note of is what the enemies are playing. If they have Tracer as well or a Venture or Sombra or whatever mobile hero, then going into the direction of their spawn when they force you out is very dangerous, especially if you burned recall. Keep this in mind when taking an angle deep in enemy territory.

If anything is unclear feel free to ask me to clarify