I would like to ask if this retention KB is still valid for LEM 6.1 (latest)see below ?
LEM's retention is sized based. So, you are right - the oldest events are purged to make way for the new events.
From the 'LEM Reports' utility, you can run the Database Maintenance report. This will show the trend of events per day, the time of the oldest and newest event, the size of the normalized data store, etc. This should help you determine if you need to increase the size of the virtual disk. The resource below is a useful reference if you do need to increase the size
Yes, it is still valid. The retention methodology hasn't changed.