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.
When your account changes from the time based free tier to the only forever free tier they often shut things down. Even if you're careful not to use the time based free tier stuff, which I was, it went pop anyway.
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.