They have live migration now (for many years) but they didn’t back in the early era. I’m not sure they set it up for the classic environment but I think they must have - in the early 2010s you could get notices that your VM’s host had failed or was about to fail and you needed to launch a new one but I think that stopped by around 2015 as I had servers running for a deprecated project which were finally shutdown this decade and it seems like rather good luck not to have any failures in 7+ years on old hardware.
I received last such email about a year ago for a VM in either free-tier or like the cheapest one available. This probably has to do with VM flavor you choose as well as with the time you created it.