It looks like your repository entries read "buster". That will stay OK.
You can check this in /etc/apt/sources.list, or in /etc/apt/sources.list.d/debian.list (not sure what Antix uses, I use pure Debian)
The message is just to inform you that Buster is oldstable now, since the new release Bullseye is stable now.
It would have been different if it states:
http://security.debian.org stable/updates This would upgrdade you system, which probably gives trouble.
AFAIK Antix (like MX) cannot just been upgraded to a new Debian release. At least, not without trouble. But correct me if I 'm wrong.
I tried (for fun) "upgrading" my MX 19 to the Debian sid release. Seemed to work OK for the Debian part, but it broke some MX utilities. And it ended in a mess. You will learn a lot from it, but if you need a stable (!) system, stay away from it.
Warning from the website
https://wiki.debian.org/SourcesList:
The 'distribution' can be either the release code name / alias (jessie, stretch, buster, sid) or the release class (oldstable, stable, testing, unstable) respectively. If you mean to be tracking a release class then use the class name, if you want to track a Debian point release, use the code name. Avoid using stable in your sources.list as that results in nasty surprises and broken systems when the next release is made; upgrading to a new release should be a deliberate, careful action and editing a file once every two years is not a burden.
Sorry for the jargon...
If it's not clear, pleas let me know.