The private messaging system was removed, as far as I know is to comply with the GDPR by preventing the collection of private data in the first place. And all messages to be made public through the forums with full-control for the sender to delete and modify its message or topic.
In Layman's term, it was removed for everyone's safety.
Regarding the Mobile Build Service, as what I've understood from Nepeo. There should be no build attempt limit, and the mobile build service is a built-in feature included in the Construct 3 Subscription.
Although there is a Mobile Build Service size upload limit.
+Compressed : 500MB
+Uncompressed : 500MB
+Compressed : 1GB
+Uncompressed : 1GB