You're not getting a key to anyone's house here. Hotels also take ID photocopies in the US, for similar reasons, but they keep them in a safe. Chances are you're trusting someone you're working for to also keep the ID secure. Admittedly I wouldn't be comfortable with that either, but that is a matter of personal preferences.

I don't believe it's reasonable to place the same trust in someone on the internet on a less than secure platform, and especially not for them to have a Discord channel where they accept IDs. There are so many more ways for a huge range of people to access information on the internet than there are to see a specific photocopy in real life.