[Tutor] Which is better style for a function that modifies a list?

[boB Stepp]

I am tremendously enjoying "Practical Programming -- An Introduction
to Computer Science Using Python 3.6, 3rd Edition" by Paul Gries,
Jennifer Campbell and Jason Montojo, c. 2017.  I highly recommend it
to anyone who needs to learn fundamental Python and/or introductory
computer science.

In its chapter on lists the authors point out that a function like

def remove_last_item(L: list) -> list:
    """Return list L with the last item removed."""

    del L[-1]
    return L

does not require the return statement since the original passed in
list will be modified in place, so there is no need for the function
to return anything.  I know from experience reading the Tutor and main
lists that this is a frequent "gotcha" that catches many fellow
learners.  So I wonder:

1)  For such a function that mutates the passed-in list is it
clearer/better style to keep or omit the return statement?  Which
reads better?

2)  From a preventative of subtle bugs perspective, if one must alter
a passed-in list, would it be better to do a deep copy of the list
passed in, mutate that, and return *that* list, leaving the original
passed-in list unaltered?  Of course the function documentation would
reflect this.

TIA!
boB Stepp