This seems like very unfriendly behaviour

select pg_drop_replication_slot('mysub');
ERROR:  replication slot "mysub" is active for PID 13065
test_database=# select * from pg_subscription;
 subdbid | subname | subowner | subenabled | subconninfo | subslotname | subsynccommit | subpublications
---------+---------+----------+------------+-------------+-------------+---------------+-----------------
(0 rows)

test_database=# select * from pg_publication;
 pubname | pubowner | puballtables | pubinsert | pubupdate | pubdelete | pubtruncate
---------+----------+--------------+-----------+-----------+-----------+-------------
(0 rows)