If your tenancy is unpaid your "always free"-classed instances can be terminated without notice in order to provide capacity for a paid tenancy when necessary. You can upgrade to paid tenancy and simply keep using "always free" resources for a $0.00 monthly bill.
I'm still on unpaid tenancy and oracle docs say that reclaim servers which are under or overutilized.
I use my oracle free servers (and one paid vps on hetzner) as automatic cloud fallbacks if my homelab is inaccessible over the internet for some reason. Which means I fall in the underutilized category as most of the time workload is under 2%. So I "fake" work to prevent reclaimation by oracle and its worked well so far.
