Dropping a row in a dataframe
The documentation for sort, to pick a common example, says:

You can also use the list.sort() method. It modifies the list in-place (and returns None to avoid confusion).

One benefit of returning the object is to chain calls. If consumers of your code expect to be able to chain calls, perhaps returning the object makes sense. Otherwise I would say the more common behavior in Python is to return None from functions that mutate their arguments.
(Other languages may have other conventions.)