Are you sure? I seem to remember Discord requiring an email but not mobile number
I know Telegram absolutely requires a mobile number, though
It will sometimes let you register without a phone number, but other times it will demand one. I imagine it has to do with IP reputation and how many accounts have been created from your network recently.
until the discord server goes down
If the thing goes down there's no guarantee any indexer/scraper is going to index/scrape it