I challenge you to find earplugs that can be worn 24h a day without any physical discomfort, provide 60dB of attenuation across a wide range of frequencies (these horns probably span low 100's of Hz to mid kHz), and are reasonably inexpensive.
Even if you could find such beasts (which I don't think you can), then you have the problem that you effectively can't use your hearing. You thought that social interactions were hard with a mask? Imagine not being able to use your hearing at all.
(yes, 60 dB is necessary because that's how much you'll need in order to sleep - the human ear has an incredibly large dynamic range and even 30 dB_SPL conversations are enough to keep some people awake for hours)
As it stands, many of the residents near these protests are being subjected to low-level sleep deprivation, which is literally torture (as in, used as such by organizations who actually want to extract information or confessions from people).
To clarify - it's not honking that's torture, it's sustained honking at a duration and intensity that will cause sleep deprivation and other psychological damage.
Completely independently of the reason that these people are protesting - this particular means is not humane.