<…> I saw that 16,117 profile has been deleted. Does anyone know in which conditions profiles have been deleted from the forum? They were inactive for a long time or any other violation? <…>
There’s this to go by:
In the forum's first couple of years, accounts would sometimes be deleted. Also, in rare cases an ID can be skipped (eg. if a database transaction gets rolled back).
The former quote points to around 16.000 missing account Ids back in August 2018. The current toll would be 16.005 (3468951 Id of most recent member – total members on Bitcointalk’s stats info at the bottom of the index page). That kind of points to the skips in Ids being very exceptional nowadays.
The most common cause might be (my speculation based on the second quote above) a rollback on an insertion record, due to resources already being locked at the time for some reason or other (i.e. reindex taking place, bot post inserts too close in time, who knows ...).