It's currently impossible to get the block responsible for a players current spawn anchoring, that if broken would break their spawn.

This is useful when creating plugins that interact with the respawn system, such as EdGruberman's sleep, whose only remaining nms dependence is getting the actual bed anchor block.

https://github.com/EdGruberman/Sleep/tree/master/src/edgruberman/bukkit/sleep/craftbukkit

You can get the expected spawn location using the current API, which is not the current anchor, and workarounds by searching for the anchor can lead to issues when multiple beds/anchors are nearby.