I went on a sort of deep dive of how swap partitions are encrypted. Most of the time the crypto block device is tied into the swap system, which makes a lot of sense, reusing layers and all that, That is how linux and freebsd do it. but openbsd is interesting. it uses 512KB blocks each with it's own encryption key, and once all references to a block are gone the key is discarded. so no guarantee but openbsd does try to lose anything that was put in swap as soon as possible.

https://man.openbsd.org/sysctl.2#VM_SWAPENCRYPT~2