Assange had first started to insinuate a connection to Seth Rich in 2016. He has had plenty of opportunities since then to share any supporting evidence, but he hasn't. That isn't due to any free speech issue.

Also Assange was still communicating with and receiving DNC documents from his source after Seth Rich was already dead. Seth Rich wasn't Assange's source.

https://www.nytimes.com/2019/04/20/us/mueller-report-seth-ri...