Getting a custom URL for a Discord server (e.g. discord.gg/hackernews) requires 14 "server boosts" which cost $35/year each, so nearly $500/year. There's a discount if you have their premium Nitro package, but even then it's something on the order of $300.

Meanwhile discord.io is free and you won't lose your URL to a crypto scam server when someone forgets to renew their boost. Kind of inevitable that such a service would pop up.