tested with 4.6.0-0.nightly-2021-06-24-211140, followed the steps in comment 0, AlertmanagerReceiversNotConfigured alert was not fired again
expr: clamp_max(sum(alertmanager_integrations), 1)
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory (OpenShift Container Platform 4.6.38 bug fix update), and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.