Edit: As I see it's true.
Source code for OCI images: https://github.com/bitnami/containers/tree/main/bitnami
If you look at the folders there, you'll see that all of the older Dockerfiles have been removed, even for versions of software that are not EOL.
For example:
PostgreSQL 13 (gone): https://github.com/bitnami/containers/tree/main/bitnami/post...
PostgreSQL 14 (gone): https://github.com/bitnami/containers/tree/main/bitnami/post...
PostgreSQL 15 (gone): https://github.com/bitnami/containers/tree/main/bitnami/post...
PostgreSQL 16 (gone): https://github.com/bitnami/containers/tree/main/bitnami/post...
PostgreSQL 17 (present): https://github.com/bitnami/containers/tree/main/bitnami/post...
> The source code for containers and Helm charts remains available on GitHub under the Apache 2.0 license.
Ofc they're all still in the Git history: https://github.com/bitnami/containers/commit/7651d48119a1f3f... but they must have a very interesting interpretation of what available means then.