I cannot recall in 31 years at Le Mans when the Safety car has not been deployed at sometime during the race.
However saying that it is only a fairly recent addition. Maybe at the same time as the introduction of the chicanes. Prior to that there was a "course car" which would circulate and drivers were expected to reduce their speeds when seeing yellow flags I cannot recall if they had to line up behind the car or not. It would only be used for the very serious incidents, then again it was pre carbon fibre and so track debris was less of a risk to tyres. Maybe also with less safety features the drivers were more aware of dangers and treated the marshalls flags with more respect.
Simon