You don't solve harassment by censoring free speech. By not including this feature, you're not just stopping users from being harassed, you're also stopping users from being able to nicely talk to each other and form friendships and affiliate relationships. It makes Itch a cold-cold, competitive commercial place, not a friendly community.
I can't send a gift to a fellow seller on Itch with a private message, for example. I can't make someone's day. And the same way, I can't receive a gift either with a nice message from someone. Or talk to someone about doing a joint promotion, or just share our experiences.
Ultimately your choice, but I thought I'd draw attention to this. I'm not the only person who feels this lack of warmth on Itch.
There are ways to create a private messaging system where the user has full control over who can contact them, and can stop unwanted users from contacting them in a private message. If you give users the control over their own private messaging life, then there's no need for moderation, really (except for following the main rules of the website/community -- the moderation that is already in place).
Sure, with more freedom comes more responsibility, and more social dynamics, but stopping people from being able to cater to their needs and express themselves is not a solution, either. You need to find a working balance -- to create a communication system that keeps users in control and safe, and that doesn't require heavy moderation, but that allows them to tend to their needs and to openly communicate with each other, even in private if they feel the need to.
What would be the harm in implementing a basic "send a private message/letter/notice" feature to a user on Itch, where they have the option to:
- Opt to stop receiving messages from this user in the future.
- Opt out of the private messaging system altogether (effectively turning the feature off for their account).
- Report a problematic user, who is breaking the rules of the website/community.