The documentation clearly states that idle vms on free tier could be reclaimed: https://docs.oracle.com/en-us/iaas/Content/FreeTier/freetier_topic-Always_Free_Resources.htm#freetier_topic_Always_Free_Resources_Infrastructure
Idle Always Free compute instances may be reclaimed by Oracle. Oracle will deem virtual machine and bare metal compute instances as idle if, during a 7-day period, the following are true:
CPU utilization for the 95th percentile is less than 15%
Network utilization is less than 15%
Memory utilization is less than 15% (applies to A1 shapes only)
So don't create a 4 core 32gb ram vm to run a vpn, and you should be fine :)