The "community spaces" opted-in to the discovery platform. It not only requires confirmation from the server owners but also that the server has specific metrics to even be elegible to enter. If the users don't want their messages to be public they should complain to the server owners instead.

The problem of "not being able to easily search Discord server" is a real problem. Not too long ago I was unable to find any information about UE4 modding and had to dig deep until I found a few discord servers centered about it. The only place of information aside from two small docs sites.