This is an animation curve showing logged rigidbody.velocity.magnitude once per Update

In OnCollisionEnter Unity will tell you the velocity of the other rigidbody

Guess what number it reported for this collision

OK, so if doing collision.rigidbody.velocity gives us the velocity _after_ it reacts to a collision then maybe collider.relativeVelocity is the relative velocity of the collision itself?

Hahahaaaaaaa nope

It seems the rigidbody velocity slowdown is happening at some point after FixedUpdate and before OnCollisionEnter, so if I cache the velocity every FixedUpdate and use that, I get accurate collision velocity rather than post-collision velocity

Oh, to clarify the initial post in this thread, Unity gives 8.5 as the collision velocity

It may be working as intended but honestly I'd call that a horrific bug