See How do I send a private message? in https://help.researchgate.net/hc/en-us/articles/14292841685009-Updates-requests-and-messages for the change. Most probably, the reason for this change were spam attacks using the Message function from fake accounts. See https://www.researchgate.net/post/Cant_access_a_paper_that_I_was_supposedly_sent/1 and https://www.researchgate.net/post/New_type_of_scam_New_approach_using_fake_accounts/1. This may have been also the reason to remove the Comments function.