Naven
A message request is how many apps handle a conversation started by somebody you have not talked to before. The message waits in a separate list, the sender cannot see whether it has been read, and nothing more arrives until the recipient accepts. Declining or ignoring it leaves the sender with nothing.
Requests matter most for children and teenagers, because unwanted contact from strangers, rather than content they went looking for, is where most harm online begins.
Where Naven stands
On Naven a conversation from somebody new arrives as a request, and Settings has a switch for who can start a conversation with you. Because there is no member directory, somebody has to know your email address, or be sent your link, to send one at all.