So patch looks good to me, but documentation changes are missing from it. UNLISTEN should be removed from the list of commands not allowed and moved to the list of those which are allowed [1]. Moreover, src/test/regress/sql/hs_standby_allowed.sql and src/test/regress/sql/hs_standby_disallowed.sql tests should be updated based on these changes in the patch. What is surprising to me is that make check-world does not fail with this change, but there is an explicit check for UNLISTEN *. So does this mean those tests are not run or does it mean that this patch does not fix the problem?
I've made the requested changes to the docs and to the regression tests.
I think that specifically the standby regression tests do not get executed by check-world - see section https://www.postgresql.org/docs/current/regress-run.html#id-1.6.20.5.8. I'm guessing this should be executed as part of the build verification pipeline for patches, but I don't know anything about the PostgreSQL build infrastructure.
Unfortunately I'm extremely tight for time at the moment and don't have time to do the appropriate hot standby setup to test this... As the patch is pretty straightforward, and since I'm hoping you guys execute the tests on your end, I hope that's acceptable. If it's absolutely necessary for me to test the patch locally, let me know and I'll do so.