If the intention is to have an internal, instance-only post, I believe such a thing could be enforced with an automoderator bot. I had a lot of success throwing the Lemmy API into an AI and generating my own moderator bot from that. Could work for you.

As an admin you can use your special admin powers to pin posts.

I would like Lemmy to have a “don’t defederate” or “only internal votes” checkbox for server-only posts, though.

In theory you could also drop external votes for an a community. Here’s an SQL query I quickly threw together to select all upvotes submitted to a community that don’t come from the server the community is hosted on:

select post_like.id, community_id, community.actor_id, voter.actor_id 
    from post
    inner join community on community.id = post.community_id
    inner join post_like on post_like.post_id = post.id
    inner join person voter on (
        voter.instance_id != community.instance_id and voter.id = post_like.person_id)
    where community.id = ?

You’ll need to change select...from to delete from to wipe the votes from the database. I imagine it’ll take a while to complete, but on smaller servers it should be feasible? You could also add something like and post_like.score = - 1 to only delete downvotes.

What is the problem with getting down votes ? Visibility? I thought Lemmy supported pinned posts.

You mean just like a specific recent post on our main about defederating another instance being drowned out by members of said instance :)