I could not find a help page on sharing research, but I assume it is the same situation as for the Message function. See How do I send a private message? in https://help.researchgate.net/hc/en-us/articles/14292841685009-Updates-requests-and-messages for this change. Most probably, the reason were spam attacks using the Share and the Message functions 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. - Unfortunately, there is no longer a simple way to share research or to send messages to users who do not follow you. Sharing research with those users is possible only when they request a research item. You may try to find out their email addresses through other web pages or through publications, but this is often difficult. On the other hand, you should think twice before contacting people who do not know you. Some may find this annoying.

It would have been beter the sharing and message links were not restricted to enhance inffomation dissemination