Is that really a problem though? Most people would just add their friends via username and bypass the whole user discovery process. Discord has demonstrated that this works perfectly fine, even with anonymous accounts not tied to emails.

I’ve only used discord a couple months in a handful of servers and I get spam dms..

that problem was solved like 25 years ago, just ignore everyone who you didn't seek out yourself by default. Basically, make liberate use of the block functionality.

Matrix seems to be doing just fine.