I think I agree with Serhiy here... the whole difference between run([a]) and run(a) is that in the first case 'a' is treated as a path, and in the second it isn't. This distinction long predates pathlib.

Say I have an oddly named binary with an embedded space in its name:

bin = Path("/bin/ls -l")

# Check that this weird file actually exists:
assert bin.exists()

# runs this weird file
subprocess.run([bin])

# Currently an error; if this is implemented, would run
# /bin/ls, and pass it the -l argument. Refers to something
# completely different than our .exists() call above.
subprocess.run(bin)