You use the in
test if you’re doing that way… hence the link to Tests
The exact filter will depend on where you place it.
{{ states.sensor
| selectattr('entity_id', 'in', label_entities('monitored'))
| selectattr('last_changed', 'lt', now()-timedelta(seconds=3600))
| map(attribute='entity_id')
| list }}
or
{{ states.sensor
| selectattr('last_changed', 'lt', now()-timedelta(seconds=3600))
| map(attribute='entity_id')
| select('in', label_entities('monitored'))
| list }}