As long as you do regulary updates of your debian stable, you are not secured against supply chain attacks.

I don't think you understand Debian. There's a new release every 2 years. A few months before every release there's the so called package freeze on the testing branch. The version the packages are on at that point that's the version they will have for the next stable release. Between releases the only updates are security updates.

Do you mean I should worry about the fixed CVEs that are announced and fixed for every other distribution at the same time? Is that the supply-chain attack you're referring to?