In #20347, I was asked to allow list-like inputs for the .str.cat methods for Series. However, I didn't know that is_listlike({'a', 'b', 'c'}) is True (for sets) until I stumbled over it in #22486.

These objects should imo clearly be disallowed, lest users shoot themselves in the foot massively:

>>> s = pd.Series(['a', 'b', 'c'])
>>> s.str.cat({'a', 'b', 'c'})
0    ac
1    bb
2    ca
dtype: object

I'm not even sure this should go through a deprecation cycle - IMO it's just a plain ol' bug that should be fixed ASAP.