Please make sure you have "undetermined" selected in your list of spoken languages in your lemmy settings in addition to any other languages you speak.
Most lemmy content does not come through with a language tag, and you need to have undermined selected as an option to see that content.
So, new or newer instances will not be able to view old posts and comments from established instances? Wouldn't this behavior discourage new instances from forming as the bedrock of content grows? This behavior suggests I should make my account on the oldest and largest instance I can find, rather than branch out to smaller, newer instances. Is that what you would recommend to new users?
They can, they just have to search for it manually. So if you see content on a group that your instance isn't aware of for some reason, if you copy and paste the URL in to the search box of your instance, it will pull the post across for you to interact with.
Not really. Aside from the fact that things like mailing lists remain useful even without easy access to their archived content, there are a few mitigating factors as well
Firstly, as I mentioned above, you can manually pull content across.
Secondly, if someone who can see the old thread replies to it, then that will force it to federate to your instance, as long as someone on your instance is subscribed to the group.