People (bots) will use it for spamming, spamming and more spamming. And people (real people) will harrass, be harrassed and take screenshots, exposing previously thought private information. Even with public chatting those issues arise. With pm that would escalate. It is high maintenance and itch lacks the advertisment revenue to moderate this. Even twi...x considers collecting payment or already does so, for various things - in addition to the advertisments.
Think try my game on discord scam transforming to try my game on itch private message scam.