Would this be considered a valid reason to apply for deferral for graduate school admission? I have been recently admitted to a top 5 graduate school in the US and I am considering deferring my admission by one year.
My reason is that I am currently pursuing a master's degree and under normal circumstances, one can complete it in 3 semesters, however, I am in a position to graduate in two semesters. Still, I would like to work on a research project with my supervisor (who is very famous in his field) in the third semester starting this fall to hopefully publish something useful.
Do you think that this is a valid reason to ask for a deferral?
You can ask the program what their policies on deferrals are, but I wouldn't defer for this reason.
Your goal at this point is to move to the next step. You've been admitted to a PhD program, so you completed that transition, it's time to start working towards your PhD.
It's perfectly reasonable to finish up previous work as a side project (you don't want to delay progress on your PhD, but it's worth dedicating a bit of extra time to polish up a manuscript and get it through review). Assuming your start time is fall 2021, you still have several months to push the project as far as you can. I would strongly suggest that approach rather than trying to buy another year, even if it's permitted to do so (it may not be).