This article is counting "secure communications" to be those that are encrypted and can be entirely self-hosted. This eliminates 3rd party snooping into communication. What constitutes self-hosting? If the software in question relies on a server that is not fully controlled by the webmaster in order to operate, then the service is not truly self-hosted.
Other factors to consider: Anonymous accounts and end-to-end encryption (E2EE).
If the communications software is end-to-end encrypted, then all the better, but that is not a strong requirement to be secure in this context.
- Email (PGP encrypted)
- Requires a domain
- Requires a (publicly announced?) mx record
- Tox - Has security concerns that have been somewhat ignored by tox development team. See issue 426.
- GNU Jami
- Nextcloud Talk (plugin for Nextcloud servers)