Some thoughts I had about magic suppression in a society like Equestria, as well as history of magic suppression. Some of this has been inspired by Enigmadoodles' fic, Shadow of a doubt and other fics where this has be touched upon.

What is Magic Suppression?
So, basically for as long as there has been unicorns, there has been magic suppression; the act of cancelling a unicorn's capability to use magic. It has usually been used to remove dangerous individuals, or help those with uncontrollable magical surges. However, suppressing magic for long period of time can lead to deterioration of mental faculties as the leylines aren't cut off, and they build up in the core of a unicorn's horn, leading to magic-induced migraines and sometimes worse. Naturally, suppressing such an innate ability is very good at causing anxiety.

How do magic suppressants work?
Horn rings, a rune-laden ring screwed directly on the unicorn's horn with tiny screws are what most think, but they tend to only be seen in fantasy novels and theathre, rather than in historical sources. Runes, however, are important for cancelling the leyline output. The rune on the magic suppressant acts as a block between the horn and outside world, but currently it still only works in one direction: Leyline energies still get in the horn, but cannot get out.

The rune in question also directs the severity of the suppression and stronger rune can allow for partial suppression, for example allow weak telekinesis but nothing else.

Wouldn't it be easier to cut off the horn?
Besides most unicorn-including societies finding the idea abhorrent, it is often seen as a very dangerous practice used by those who do not actually know their anatomy: damage (like Tempest Shadow's) to the horn's core doesn't stop the performing of magic, only makes it less precise and more volatile. Removal of the horn would make the unicorn in question a ticking time bomb. Hence why suppression became a thing to control those deemed dangerous for whatever reason.