Uhm. Do you argue for itch to reinvent facebook? Because it sure reads like that.
This discussion is pointless. There are "solutions" offered. But there a no technological problems here. Any run of the mill out of the box message board has private messages. It is not a technical issue. You do not need to provide solutions for problems that are non existant.
It is a money problem. Again, not a technical problem to implement such features.
It is staff problem. You need people to deal with issues that grow with usage. And using the feature will not create income, since itch does not run ads on page. On a thing like facebook, the more you use their messaging features, the more advertisement revenue there is, and they can pay staff to sanitize spam, scam and harrasments. Anarchical self service protection is not enough for a legit site, not even with a disclaimer.
You should ask youtube, why they abandoned their private messaging. If they do not have the money to run such a service, what makes you think itch would?
https://support.google.com/youtube/thread/12446824?hl=en
Oh, and add imdb to that club. They kicked their whole message board out. Though I do not remember if they had private messages. You can still access it on https://moviechat.org/ , they picked that up, but there is not as much traffic as before - and of course it is ad financed.