Basic shift cipher in Python
Bug


elif ' ' or '/t' or '/n' in letter: 
    new_message += letter



Provided that execution reaches that point (i.e. letter.isalpha() is false), this condition always evaluates to True, because the space character is a non-empty string.  The rest of the expression doesn't matter due to short-circuit evaluation of or.  For the record, the string consisting of the two characters / and t is always True, and the two-character string '/n' can never appear within a one-character string.

Of course, that means that the elif letter.isnumeric() and the else branches are unreachable.

Did you mean to write this instead?

elif letter in ' \t\n':
    new_message += letter


Naming

letter might not be a letter of the alphabet.  A better name would be character, char, or just c.

Design

shift would be a better as a pure function: it should accept the message as one of its parameters (instead of prompting for it) and return the enciphered result (instead of printing it).  Then, you would call

print(shift(input("Input message you would like encrypted:\n"), -1))


Why do you want to disallow certain characters?  If digits and spaces are to be passed through unmodified, why shouldn't the same be true of punctuation?

Efficiency

Strings in Python are immutable.  Every time you call += on a string, it allocates a new string and fills it with the old content and the new ending.  Therefore, += is a relatively expensive operation and should be avoided.

A better alternative would be to build a list (by appending or using a list comprehension).  Then, you construct the result by calling ''.join(…).

Pythonic cleverness

Your alphabet could just be string.ascii_lowercase instead of that list.

The classic way to implement a Caesar Cipher in Python is using str.translate().

from string import ascii_lowercase as ALPHABET

def shift(message, offset):
    trans = str.maketrans(ALPHABET, ALPHABET[offset:] + ALPHABET[:offset])
    return message.lower().translate(trans)

print(shift(input("Input message you would like encrypted:\n"), -1))