I recently upgraded to Ubuntu 20.04 and therefore to Ark version 19.12.3. It turned out that I can't any encrypted Zip files without AES256, AES192 or AES128.
This also means that the warning "This encryption method may not be supported by older archiving software" is always shown.

So either something is missing (in that case the user should be informed what he can do to enable backwards compatible encryption) or it is just broken because it's missing the legacy encryption options.