the following works in Linux

import os
os.execl('/bin/bash', '')

doesn't in mac:
>>> import os
>>> os.execl('/bin/bash', '')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/me/.pyenv/versions/3.6.4/lib/python3.6/os.py", line 527, in execl
    execv(file, args)
ValueError: execv() arg 2 first element cannot be empty

works if you add a space
>>> os.execl('/bin/bash', ' ')

notice the space in 2nd argument.

technically it is also possible to run a command without arguments - why not allow for the case where *args is []?

>>> os.execl('/bin/bash')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/siyuan/.pyenv/versions/3.6.4/lib/python3.6/os.py", line 527, in execl
    execv(file, args)
ValueError: execv() arg 2 must not be empty
>>>

actually just learned that argv0 is program name.

in that case is that because of a platform difference where in macOS it doesn't allow for program name to be '' and in linux it does?

Which exact python version did you test?  The current codebase raises this exception on all platforms, not just macOS (see os_execv_impl in Modules/posixmodule.c)

The behaviour of raising an exception was introduced in issue 28732 for Windows, not sure when that code was refactored to raise the exception on all platforms.

The behavior was changed in Python 3.6 for all platforms to catch this error; see Issue28732.