Looks like the #GitHub "abuse" team "cleaned out" the #matrixnotorg account and "cleaned up" the #Matrix.org #security issues..
https://github.com/matrix-org/matrix.org/issues/367#issuecomment-482649539

Good web.archive.org exists.

GitHub, you #failed.

Please restore discussion on the Security threads · Issue #367 · matrix-org/matrix.org

It will be a good place for discussion of security. Locking down topics seems to indicate a lack of transparency and doesn't inspire confidence

@erAck
Thanks for sharing this. I wasn't sure whether I could trust Arathorn's comment on HN at first but it restores some of my faith that he found those archive.org links for us. https://news.ycombinator.com/item?id=19646843
we (Matrix.org) haven't deleted the issues; we were deliberately leaving them up... | Hacker News