There are many special achievements in "Silent Hill 2: Remastered (SILENT HILL 2)", and now there is no way back is one of them, and the requirement to achieve this achievement is to try to leave Silent Hill in the observation deck area. As long as there is the movement of walking back, it will be fine.
How to get the No Turn Back achievement in Silent Hill 2
There is no turning back now
The requirement to complete the achievement is to try to leave Silent Hill in the Observation Deck area.
After arriving at the South Valley, turn around and try to leave. As long as you have this action, you can complete it. I don't know if it will be useful to come back later, so it is best to do it immediately when you get there.