What was that short barrel at the end of old USB cables, and why are they gone now?

That thing is called a Ferrite bead or ferrite core.

Simply put, any wire can act as a radio antenna. The radio waves that it picks up are then transmitted to the device, which might interfere with the signal.

The ferrite bead filters out some of that electronic “noise.”

If the wire is short, shielded, or shitty, it may not have one, but they are still in use on some quality cables