Can you elaborate on this? These are not preemptible, why would they disappear without warning? I've been running on these for 2 or 3 years without any issues thus far.
If you're below their "idle system" threshold, they'll shut it down.
Get past the 3 month or whatever it is tier. Put a low level load on the system to get it above the idle threshold and you should be good to have a nice personal playground. Probably.
I've had systems go down to both these. They also have rejected every credit card I tried to throw at them. Which is a known problem for them as well as some other cloud providers. I no longer put anything critical on OCI.