Re: backend hangs at immediate shutdown (Re: Back-branch update releases coming in a couple weeks)

Here, "reliable" means that the database server is certainly shut
down when pg_ctl returns, not telling a lie that "I shut down the
server processes for you, so you do not have to be worried that some
postgres process might still remain and write to disk".  I suppose
reliable shutdown is crucial especially in HA cluster.  If pg_ctl
stop -mi gets stuck forever when there is an unkillable process (in
what situations does this happen? OS bug, or NFS hard mount?), I
think the DBA has to notice this situation from the unfinished
pg_ctl, investigate the cause, and take corrective action.

So you're suggesting that keeping postmaster up is a useful sign that
the shutdown is not going well?  I'm not really sure about this.  What
do others think?

I think you are right, and there is no harm in leaving postgres processes in unkillable state.  I'd like to leave the decision to you and/or others.