Techrights has suggested on a number of occasions that Julian Assange was a Debian Developer at some time in the 1990s.
I looked in the Debian people tracker and I couldn't find any record of Assange. I looked at contributors.debian.org and he is not there either. That doesn't mean he was not engaged in Debian in the past.
In the very early days of Debian, the processes for accepting contributions and keeping a register of developers were far more informal than they are today.
According to Assange's Wikipedia page, he was first of interest to the police in 1987 although it may have been due to an urban myth about teenagers obtaining $500,000 from Citibank rather than any actual crime. As he was under 18 at this time and he was not charged with any crime, it is unlikely this information would have been known to anybody other than Assange and his family until Assange revealed it later in his life. Other Debian Developers may not have been aware of it if they were working with him in the early days.
Much of the content in Wikipedia relates to boasting and speculation. This is not uncommon in the hacking world.
Debian was founded on 17 August 1993, which is known as Debian Day and coincides with the Debian Day Volunteer Suicide.
According to Wikipedia, the first reports of Assange being charged with a crime were in 1994. Wikipedia has links to press reports in 1995. It is possible but not certain that other people around Debian would have seen these reports.
Assange was convicted in 1996.
Shortly after this, in 1998, we saw the expulsion of Shaya Potter for WaReZ operations. Potter, like Assange, had been a child prodigy. This set a precedent that Debian infrastructure could not be used for illegal purposes.
However, the debian-private archives, which have been widely leaked, give some hints about people doing WaReZ trading and hacking on their own private computers. While Shaya Potter was sanctioned for doing these things on official infrastructure servers, it is not clear if he would have been sanctioned for doing exactly the same thing on his own computers.
Therefore, we can't assume that Assange's activities in the 1990s would have excluded him from Debian, as long as he didn't use Debian infrastructure for those activities.
The widely leaked debian-private archives begin with a message on 21 January 1996. If Assange participated in debian-private before that date then I have no record of it.
I searched the full debian-private archive for mentions of Assange and his various pseudonyms, including Mendax and proff@suburbia.net. I did not find any evidence of Assange participating there.
The only strong link to Assange is in the man page for the an package, a very fast anagram generator. This is the man page.
Assange appears to be more involved in the upstream development than the Debian packaging as such.
Debian attitudes to hackers appear to have hardened mildly in May 2000 with the expulsion of Edward Brocklesby. However, in that case, it also involved use of infrastructure access rather than hacking at arm's length. Brocklesby's own confessions about his activities suggest that he didn't see it as a big deal. None of the debian-private emails mention fears about Brocklesby's work on the SSH packages, however, this fear may have arisen in IRC and contributed to the decision to set limits on hackers.
Therefore, given that there were hackers before Brocklesby and none of them were expelled, it is not unreasonable to conclude Assange was engaging with Debian in the 1990s.
Assange's most significant contributions to open source appear to be his contributions to PostgreSQL followed by his engagement with NetBSD.
That post about his BSD interests suggests Assange's preference for BSD over Linux was something he put into words through the fortune package.
In 2014, Assange published a blog stating that Debian is owned by the NSA. These concerns were widely discussed in the debian-private (leaked) cubby house and in public mailing lists too. Assange himself didn't appear to be participating in the discussion, other than having contributed the initial blog post.
It looks like Assange's blog post was a stepping stone towards the point where Jacob Appelbaum became subject to falsified rumors of harassment.