As of the 1.16.2 snapshots, there is a new option in the /spawnpoint command that allows for the specification of a y-rotation rotation along with the spawn point coordinates. However, unlike all other situations where rotation is used, there is no option to specify an x-rotation as well.
Now, I'm not strictly speaking sure if this would be considered a feature request or a bug (on account of the resolution of MC-195052 seeming to suggest that the angle argument in spawnpoint is its own thing), however due to the inconsistency with other commands I would imagine that x-rotation was simply looked over, hence my report.
Can confirm