I have mixed feelings about this.
There are times where it would be nice to contact someone in private. But then I imagine what this feature would be used for. And I have seen many scams in open message system. Especially developers get scam offers often, that are tailored for small business. This one here should be a warning for all small content creators, not only games: https://pbfcomics.com/comics/hacked/
If the developer has no contact info, no socials on profile and project, maybe that developer does not want to be contacted.
That being said, if there is a project and that project has ratings enabled, you can write a review that is only shared with the developer, if your reviews are private.
For general collabs there is a message board here.
And if I remember correctly, some sort of profile message board might be considered for the future.
(oh, and if that project is licensable, there might be contact info in the downloaded project in a readme or some such)