The comment in the Gemfile says:

This should be part of "json". We can remove this when "json"
depends on "ostruct" explicitly.

But that's not very clear, since this gem doesn't have a dependency on json that I can find.

It's possible (likely, I think, since I have seen it before) that some other dependency (likely one used in testing or generating documentation) required ostruct without explicitly depending on it, and it has likely since been fixed.