This seems like a good safeguard. On a site with 100k users there’s a big difference between “I was just chatting publicly with this person and want to continue privately” versus “This person I’ve never heard from just sent me a message” — we have flags, but flagging is a bit harsh if someone you don’t know just sent you “hi” and you’re not interested (how long do I have to talk to this person only to find out that they’re spam? sometimes it’s a while).
Discord does this, and I find it to work well in situations where you’ve joined a server with many people.
In their case they have a friend system, which is how they specify “members you may not know” so that doesn’t apply to us. Maybe the trust level system is a reasonable analogue? i.e., “filter direct messages from new members”?
The first time you receive a message from someone, they give you buttons to accept/ignore/report (on the lower right here, if you’re unfamiliar with their layout):
Some more detail on this feature from their support site: https://support.discord.com/hc/en-us/articles/7924992471191