If you have been running testing while Stretch (9) was stable you were indirectly following Debian Buster (10). Since debian 10 Buster became stable today July 7th, those using testing instead of Buster repositories will be automatically jumping to the next release, called Bullseye (Debian 11). The official Devuan repositories automatically “merge” debian and devuan packages, so testing will remain Buster till Devuan releases 3 (Beowulf) and therefore testing will become Devuan Chimaera. If manually you have added Debian testing be careful of incompatibilities. (some people do this, will swap Devuan merged for Devuan/devuan, and add a debian repository directly).Antix is more clear on its repositories, separating antix and debian and using other configurations to block systemd related packages from installing or being indirectly dragged as dependencies. Since Bullseye is an all new appearance, the repository was opened to public just today, one has to be extra careful while merging packages between the known and the unknown entity. Antix following Stretch has had the label 17, Buster has the edition 19 (a beta version has been issued. Get the iso from here, more information can be found in the Full announcement), and I suspect Bullseye will be Antix 21.
Some people think that the order of stable, testing, sid in Debian is just packages that are closer to upstream or have been given more time to be tested before they become stable. That is not the case, Debian induces a new architectural structure and guidelines with each release and therefore how packages interrelate and interdepend with each other changes many more things than just edition numbers.
Many people, myself included, have dared and run sid (unstable) for years but had no problems. This does not mean that testing (Bullseye) will be stable and easy on its first day. Buster becoming stable is a known entity people have tested for two years to reach stability. Some forks of Debian are solely based on testing. What is new today is Bullseye. Bullseye is neither Buster nor is it Sid, it is its own animal and to people other than Debian developers and packagers a totally new distribution.
Our advise for stability would be to stay with Buster till Antix or Devuan announce that they have worked through the corresponding repositories. Antix is much simpler, and the majority of its own packages are the same for stable,testing, and sid, but Devuan remains a mystery hidden in the code of Amprolla and its way to merge its own with Debian”s repositories.
Be cautious, and find out what it is that you are doing before you engage into an upgrade that “Busts” your system. Again for a while Buster was testing, now debian testing is Bullseye.
Sometimes I get nostalgic and run Debian Wheezy (7.11 the last official edition without systemd) in a virtual machine, just so I can remember how Debian used to be and what it has turned into now.