I've been mistakenly banned from Discord before and I know from experience that pretty much any low level mod has a complete and readibly accessible history of all of my posts across all servers complete with timestamps and IP addresses

I'm also pretty sure phone number are required for sign up

I think your second point is the more likely explanation. Any other platform that would've hosted this many communities dedicated to drugs, cybercrime, etc would definitely have faced serious legal challenges. It seems much more likely that feds find it a useful platform to keep around