It seems like it would be possible to display a beacon above the world height limit, because rain does so already, and I'm assuming they're the same type of effect.
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this still affects the current version, and I don't think it is intended. Previously beacon beams went infinitely high above build height so they could be seen from much further away, but when Mojang added the ability to tint the beam's color with stained glass they seem to have limited the beacon height. This was not in any changelog and should probably be considered a bug.
Intended. See MC-67975