Sometimes extensions were made to integrate with each other, and so they can mention it as part of the Composer suggestions. If one stops being supported, but both still work, it makes sense to keep the integration in the source code, and therefore the suggestion as well.
For the specific use case visible in your screenshot, suggesting translations, I think this is no longer a very useful use case because we now have robust language packs that translate most popular extensions, so users of the extension should be able to find translations without too much trouble. Here I would indeed remove the language packs from the suggested list, even more so if those are abandoned language packs that have been migrated or replaced by the flarum-lang project.
In the past there were often multiple complementary language packs so suggestions made sense to find the package that did contain the specific extension.
