I can't seem to @mention users. This was a feature I used, as a Marketplace Vendor, extensively on Answers. Doesn't look possible now, why?
We don't have this functionality yet. Changes from Answers are outlined in the known issues article here and may be helpful: https://community.atlassian.com/t5/Feedback-Forum-articles/Atlassian-Community-known-issues-and-changes/ba-p/458302