I've never used a mailing list that didn't possess a centralized address like foo-discuss@example.com. And these all require explicit approval to submit messages to the list, in order to keep out spam.
No, they do not all require explicit approval to keep out spam. In GNU Mailman, this is a config option: you can allow non-subscribers to post or not.
All of the mailing lists that I operate on my own mailing list server allow non-subscribers to post. Due to my anti-spam configuration, this isn't a problem.
Traditional mailing lists, before the rise of spam, were usually this way.
And anyway, this is a separate issue. A list which does not re-mail postings from non-subscribers can nevertheless not do Reply-to: munging. So once you are on the list and participate in discussions, you're still sending messages to the list, as well as directly to those in the discussion.
Earlier this year I was involved in a mailing list discussion in which one of the parties was actually (unbeknownst to me for a while) a "persona non grata": someone banned from posting to the mailing list. His postings were not being seen by the subscribers, but only those in the debate. This list does use Reply-to:; he just (trivially) circumvented it.
