Alert notification receiver created through web console creates receiver with field match which is deprecated instead of matchers and when match is changed to matchers causes Alertmanager pods to crashloopbackoff state throwing the error:
~~~
ts=2023-11-14T08:42:39.694Z caller=coordinator.go:118 level=error component=configuration msg="Loading configuration file failed" file=/etc/alertmanager/config_out/alertmanager.env.yaml err="yaml: unmarshal errors:\n  line 51: cannot unmarshal !!map into []string"
~~~

Always 

1. Create alert notification receiver through web console.
Administration-->configuration-->Alertmanager-->create receiver-->add receiver

2. Check the yaml created which would contain route section with match and not matchers.

3. correct the match to matchers and not change the matchers defined like severity or alertname correctly  .

4. Restart the Alertmanager pods which leads to crashloopbackoff state.

Alert notification receiver uses match field

Alert notification receiver should use matchers filed